Adding ability to skip cert verification on HTTPS checks

[hairyhenderson]

This adds support for a new environment variable
CONSUL_CHECK_HTTP_SSL_SKIP_VERIFY, which will cause TLS cert
verification to be skipped when performing HTTPS service checks.

Right now, if I try to do an HTTP check against an
https://<private_ip> URL, the check will fail with something like:

... x509: cannot validate certificate for x.x.x.x because it doesn't
contain any IP SANs

Sometimes it's not possible to use the FQDN that the cert is valid for,
because it may be (for example) fronted by a load-balancer, whereas we
want to check the particular instance of a service.

Signed-off-by: Dave Henderson dhenderson@gmail.com

[slackpad]

Hi @hairyhenderson thanks for the PR! Instead of an environment variable that applies to all checks it seems like this would be better as a new Boolean field in https://github.com/hashicorp/consul/blob/master/command/agent/check.go#L43 so it could be configured per-check. If the sense of the field is "skip" then it will default to false for existing checks and do the right thing.

[hairyhenderson]

Thanks for the feedback, @slackpad - that's a good idea. I'll incorporate that :)

[ryanuber]

Thanks @hairyhenderson, appreciate the contribution! I'm going to close this in favor of #1984. I agree with @slackpad that this fits better as a parameter of the check definition.