Allow users to create a rich network topology

Fixes bug 1146926

This commit enables Quantum Router support to create a router
interface with a specified IP address. Previously we can connect
one network (subnet) to only one router and a topology we can create
is limited. By this commit we can connect one network to multiple
routers and create a rich network topology through Dashboard.

This commit also fixes the folowing bugs:
* subnet_id in "Add Interface" form and network_id in "Set Gateway"
  form should be required=True since they are required fields.
* An error message is display twice when the above two forms.

Change-Id: Ifb90f70a4468fdf4e8ba356080cd0333f10e595f
Merge "Allow users to create a rich network topology"