POC: Setting upstream vhost for nginx.

[mtanski]

In the current k8 ingress there's no way to control "Host" header sent to upstream server; it's always the configured vhost.

It's desirable to support custom Host header when proxying for a number of cases. One such case is forwarding to an Service that points to an external host that expect a host paramater (like AWS S3 buckets). There's a number of others.

By default nginx set Host to the upstreams domain name (and not the one passed in) like K8 always forces. This provided an escape hatch to this behavior.

This is a proof of concept PR for review, feedback. I can incorporate all of that; I just need to make sure I'm going down the right path.

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[k8s-reviewable]

This change is 

[mtanski]

Stuck in getting the CLA signed due to LF CLA issues. But in the mean time I would appreciate the review of the work/direction.

[aledbf]

@mtanski apologies for the delay. This feature makes sense in many scenarios.
What you are doing is ok. Please finish the annotation parser so we can test this

[mtanski]

@aledbf I'm going to be away for a week; so I'll make those changes when I'm back and solicit feedback then.

[mtanski]

Have signed the CLA (org & employee). This was delayed due to some LF internal issue that prevented my org for being able to sign the CLA.

[mtanski]

Finally had a chance to get back to this. I have rebased this against master (for ez merging) and added the missing annotation implementation file.

[mtanski]

@aledbf any chance to get this into upstream?

[aledbf]

@mtanski please rebase

[coveralls]

Coverage increased (+0.007%) to 43.688% when pulling 2120aab on adfin:upstream-vhost into 7eb2b81 on kubernetes:master.

[mtanski]

@aledbf here ya go

[aledbf]

/lgtm

[aledbf]

@mtanski thanks!

[rikatz]

@mtanski Can we just update the docs with this new directive? Thanks!!!

[mtanski]

@rikatz Yes. I will send a PR this week; most likely in the 2nd half.