webadmin: Refactored more code into NeutronAgentModel

Added the code concerning the Neutron plugin type into
NeutronAgentModel, as it seems to be common to all the pieces of code
utilising NeutronAgentModel. Added the derived class
HostNeutronAgentModel, some of whose fields are not changeable. Also
repaired some minor behavior glitches.

Change-Id: I6c4bc0c6aa3a27d7ef6a5b9b5e3fa5007f332561
Signed-off-by: Lior Vernia <lvernia@redhat.com>