Fix issues causing tempest.api.network.test_floating_ips to fail

-Override create_router to give the Neutron and Midonet routers the
same ID.
-Connect tenant router to provider router when creating a tenant
router with an external_network_id.
-Unlink tenant router from provider router before attempting to delete
the tenant router.
-When transferring a floating IP from one port to another, delete the
old FIP-port association before creating a new one.

Fixes: MN-881
Signed-off-by: Brandon Berg <bberg@midokura.jp>