webadmin: Warn user if KeystoneAuthUrl wasn't configured

When testing the connection of a provider that was marked as "Requires
Authentication", I added a check whether the KeystoneAuthUrl was
configured in case the test failed. If it wasn't, the user is hinted
that this might be the cause of the failure.

Change-Id: Ie7c0a8586b4845efbee0d599b31d92ce8158d336
Bug-Url: https://bugzilla.redhat.com/1017538
Signed-off-by: Lior Vernia <lvernia@redhat.com>