Render correct Neutron L23 backend on Networks tab

- fetch 'net_l23_provider' field value from DB
- form new item on Networks tab (l23_provider)
- if net_l23_provider (OVS, NSX) was set show it instead of segment_type
  (GRE, VLAN)

After this change title on Networks tab will look like "Neutron with NSX" or
"Neutron with OVS"

Change-Id: Ief7462097960716ff55c22572751191826f2ecc4
Closes-bug: #1347687
Merge "Render correct Neutron L23 backend on Networks tab"