webadmin: Fixed provider name validation

Frontend only made sure that the name was non-empty, while the backend
enforced ASCII characters; now they're the same.

Change-Id: I552ab44967789c9ffbbd4b05758a36223da17d43
Bug-Url: https://bugzilla.redhat.com/987294
Signed-off-by: Lior Vernia <lvernia@redhat.com>