webadmin: add Network Provider tab to add host

Includes similar logic to that in ProviderModel concerning the plugin
type, so was reused using NeutronAgentBehavior and
NeutronAgentSection.

Change-Id: Ib65de6d8dd4422f4ab4b8fd6ca39684bf4f7b5d5
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>