Add partition identity to envoy deployment

[andrewstucki]

Changes proposed in this PR:

Gateways deployed in non-default partitions weren't properly identifying themselves to xDS servers. This adds the required metadata into the envoy bootstrapping file to make sure that the envoy instance identifies itself properly.

How I've tested this PR:

Unit tests + verified manually.

Checklist:

• Tests added
• CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)

[mikemorris]

Could we add an e2e test for a non-default partition when running with Consul Enterprise? Aside from that fix LGTM.

[andrewstucki]

@mikemorris that may take a bit of time since I don't believe any of the e2e testing code has any sort of partition awareness. Do you mind if I try and knock that out as a follow-up? This is unit tested well enough with the actual bootstrap template checking that I'm confident we're specifying the needed parameter that was missing previously, and we did vet this on a real k8s deployment configured for a secondary partition.