Router API Tests Enhancements

Add test to creation of router with snat rule
  - Create a router with default attributes
  - Verify the default snat attributes
  - Creating a router using admin priviledges for a tenant
     with snat value True/False
  - Verify routers for snat value True/False by overwriting
     through admin tenant.

Change-Id: Icf2f56f7aa7a2dc736c57daad6d2d5c37e0d9e32
Merge "Router API Tests Enhancements"