Add ServerName to Vault Agent template config

[pbar1]

Reopening the closed #9401 here:

When preparing the consul-template Vault config, the provided
TLSServerName (in the Vault agent config as tls_server_name) was not
being passed along to the consul-template Vault config.

Using the VAULT_TLS_SERVER_NAME environment variable was working since
the consul-template config building does consider it.

To fix that, pass it along when setting up the consul-template
config. Add a test which verifies basic TLS connectivity to Vault,
including passing along the server name.

Fixes #9183

[elsesiy]

@ncabatoff @calvn Any chance we can get this into 1.7.2? We're using the agent injector and the SNI info is not propagated due to this issue. Thanks!

[jasonodonnell]

I tested this and it worked as expected, however, I saw weird behavior where auto-auth would hang indefinitely if a bogus SNI was set. I'm looking deeper into this but don't think this is a blocker on this specific PR.

[jasonodonnell]

Holding off on merging as we look into a possible bug uncovered by this.

[jasonodonnell]

@pbar1 We're ready to merge this but a CHANGELOG file for this change is needed. Once added we'll merge and backport this to the 1.7.x branch for the 1.7.2 release.

Something like

```release-note:bug
agent: Fixed agent templating to use configured tls servername values
```

Thanks!

[pbar1]

Hey, I've been out for a few days. Sure - I'll add that changelog message.