webadmin: Refactored code concerning Neutron agent

This code would be common with the soon-to-be-added Network Provider
tab in HostModel. Refactored into NeutronAgentModel on the model side
and into NeutronAgentWidget on the view side.

Change-Id: Ifbec5113f146e4955945d6913c5b11eb60ca2389
Signed-off-by: Lior Vernia <lvernia@redhat.com>