Support AllowedCN and AllowedHostname through config file

[clwluvw]

What would you like to be added?

Allow setting AllowedCN and AllowedHostname tls fields through the config file for peer transport security.

Why is this needed?

According to the documents, flags cannot be mixed with a config file:

Caution: If you mix-and-match configuration options, then the following rules apply.
Command-line flags take precedence over environment variables.
If you provide a configuration file all command-line flags and environment variables are ignored.

[jmhbnz]

Re-opening for backport to release-3.5.

[jmhbnz]

Discussed during sig-etcd triage, closing backport and changelog completed.