Description of problem:
There's not sanity checking, or defined rules for that matter, for what is acceptable in a username regarding underscores. A username of "______" is valid, but not desirable.

Version-Release number of selected component (if applicable):
Dev

How reproducible:
Easily, always

Steps to Reproduce:
1. Go to zanata/account/register
2. Enter in a username of ______
3. tab to another field

Actual results:
No error shown

Expected results:
This case should not be acceptable.

Additional info:
The registration form can be completed, such that a new user with username "______" is created.