Remove routers_client from manager._create_subnet

This is to:
1. remove routers_client from manager._create_subnet
   because routers_client is not used for creating subnet.
2. rename _create_subnet to create_subnet for it is used
   by several testcases.

Change-Id: I102c3b01abe0ae752196d27893d43e64acfb57a6
3 files changed