keeper: support different auth methods

[sgotti]

Now stolon by default requires md5 password authentication for superuser and replication connections.

We could improve it to accept different authentication methods starting from the simpler ones like trust, md5, password, pam.

A solution will be to add some keeper options --pg-su-auth-method and --pg-repl-auth-method. These options will basically affect the pg_hba.conf generated replication entries and the user creation when initializing a new pg db cluster.

[emdem]

@sgotti Which auth methods would you like to support initially? I was thinking trust, password, md5...

If that seems reasonable, I will test my implementation and make a PR. If not, please let me know which other auth methods you think would be reasonable for a first implementation.