webadmin: Fixed buggy validation of NeutronAgentModel

Half the fields were disregarded even if they weren't valid, and the
host validation was actually too harsh (but since its result wasn't
regarded, it didn't block creation of providers).

Change-Id: Ia6f258fb7a95a9de4b5bd2d4b71c2f8327bb1dcc
Signed-off-by: Lior Vernia <lvernia@redhat.com>