Make test_l3_agent._prepare_router_data a module function

Remove an unneeded class variable, allowing _prepare_router_data
to be changed to a module level function. This allows the function
to be reused by other tests cases.

Change-Id: Ia4de3c535c703fbb2528e6cc9594bcd34f50aca1
Merge "Make test_l3_agent._prepare_router_data a module function"