Rhbz981085 Use a more human readable instruction for usernames

Underscores were missing from the error, and nobody reads regexes to
understand what and how many characters should be in a username.
Nobody.
Rhbz981085 update functional tests
Merge pull request #392 from zanata/rhbz981085-username-input

Rhbz981085 Use a more human readable instruction for usernames