1113629 - Allow usernames to contain '.'

https://bugzilla.redhat.com/show_bug.cgi?id=1113629

Addresses the ability to have a '.' in the username.

Switched validator for --login to use id_validator_allow_dots instead of id_validator.  Since id_validator has test coverage and it didn't appear id_validator_allow_dots did, I added test for id_validator_allow_dots to maintain test coverage for the --login option.
@barnabycourt you are looking at the pr. We moved this over to master.

https://github.com/pulp/pulp/pull/1299