Creates new "prepared-query" ACL type and new token capture behavior.

[slackpad]

Prior to this change, prepared queries had the following behavior for
ACLs, which will need to change to support templates:

1. A management token, or a token with read access to the service being
   queried needed to be provided in order to create a prepared query.
2. The token used to create the prepared query was stored with the query
   in the state store and used to execute the query.
3. A management token, or the token used to create the query needed to be
   supplied to perform and CRUD operations on an existing prepared query.

This was pretty subtle and complicated behavior, and won't work for
templates since the service name is computed at execution time. To solve
this, we introduce a new "prepared-query" ACL type, where the prefix
applies to the query name for static prepared query types and to the
prefix for template prepared query types.

With this change, the new behavior is:

1. A management token, or a token with "prepared-query" write access to
   the query name or (soon) the given template prefix is required to do
   any CRUD operations on a prepared query, or to list prepared queries
   (the list is filtered by this ACL).
2. You will no longer need a management token to list prepared queries,
   but you will only be able to see prepared queries that you have access
   to (you get an empty list instead of permission denied).
3. When listing or getting a query, because it was easy to capture
   management tokens given the past behavior, this will always blank out
   the "Token" field (replacing the contents as <hidden>) for all tokens
   unless a management token is supplied. Going forward, we should
   discourage people from binding tokens for execution unless strictly
   necessary.
4. No token will be captured by default when a prepared query is created.
   If the user wishes to supply an execution token then can pass it in via
   the "Token" field in the prepared query definition. Otherwise, this
   field will default to empty.
5. At execution time, we will use the captured token if it exists with the
   prepared query definition, otherwise we will use the token that's passed
   in with the request, just like we do for other RPCs (or you can use the
   agent's configured token for DNS).
6. Prepared queries with no name (accessible only by ID) will not require
   ACLs to create or modify (execution time will depend on the service ACL
   configuration). Our argument here is that these are designed to be
   ephemeral and the IDs are as good as an ACL. Management tokens will be
   able to list all of these.

These changes enable templates, but also enable delegation of authority to
manage the prepared query namespace.

[sean-]

Small phrase change:

capture management tokens in Consul 0.6.3 and earlier.

[slackpad]

Good call - that's clearer.

[ryanuber]

@slackpad left minor comments. My biggest question here is what happens with the DNS interface if the tokens are eventually going to go away? It seems very restrictive to only allow the client's global ACL token access the PQ's. I suppose you could handle it by enforcing convention, where e.g. private-*.query.consul is hidden from DNS, but you also lose the ability to revoke access by ACL token. Also a bit strange that a user who creates a PQ and can query it from the API might have to jump through hoops to get someone with a management token to enable DNS access for the new query. The DNS interface is probably the most powerful way to use the PQ's so just want to make sure we have thought this through completely.

[slackpad]

Incorporated all the feedback so far. I need to write the new ACL Guide documentation for prepared_query and then I think this should be good to ship.

[slackpad]

One other thing I need to add - the new prepared_query ACL needs to be applied to the query name as well.

[slackpad]

Squashed all the commits and reworded the initial big one with the details (since it was incorrect). My last change here will be to make the ACLs apply to the query names, not the service names. This was my mistake.

[slackpad]

Got the docs in shape to define the final state of things, now need to update the implementation to match.

[ryanuber]

Should this test be called GetACLPrefix?

[slackpad]

Yep - good catch.

[ryanuber]

@slackpad the docs are looking great and nothing stuck out to me - merge when you are ready!

[sean-]

DNS has a syntax, but it's an interface into Consul. How about:

given the limited entry points exposed by DNS.

?

[slackpad]

sounds good - fixed

[slackpad]

Got all the doc comments worked in - merged it!