Remove created routers as part of test cleanup

The network API routers test currently creates a total of 10 routers
(the default quota limit) but only deletes them in bulk during tearDownClass().
This can create issues hitting quota limits on setups without isolated
credentials, or environments that may already have routers created outside of
Tempest. This associates a cleanup with each created router and ensures each
individual test deletes the routers it creates.

Change-Id: I7f0e20cfe483fe3aa55d4641ca92ad9947fffbcc
Closes-bug: #1265498
Merge "Remove created routers as part of test cleanup"