Fix logging defaults to use a single directory and expose barman groups

So that users of the role can create just one log directory and set the
user's group such that it can write/read it.
Add ability to set an authorized key for the barman user

For use on the postgres master to rsync the archives
Update example rsync command to use for archiving
Remove SSH defaults and force the user to specify them
Merge pull request #2 from modcloth-labs/cleanup

Cleanup