Populate RHN page username field

If a username has previously been entered, use that instead of a
blank entry box.

Change-Id: I412137101cb7dfc10d8b3491719827e67c715592
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1098580
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>