Added support to verify extra-routes deletion
Added support to verify extra-routes deletion

Earlier the test was only verifying presence of extra routes
on the router.

Tempest client provides a method to delete the extra routes.
Assertion on this deletion was not present before, hence this
support is added.

Now the testcase will assert,

1. Presence of extra routes on the router after creation.
2. Absence of extra routes on the router after deletion.

Change-Id: I6f2c2ebdce227705f073a43ff04c8bddc4ca5fd4

cisco-openstack/tempest,vedujoshi/tempest,zsoltdudas/lis-tempest,bigswitch/tempest,sebrandon1/tempest,masayukig/tempest,Tesora/tesora-tempest,Tesora/tesora-tempest,cisco-openstack/tempest,sebrandon1/tempest,Juniper/tempest,Juniper/tempest,openstack/tempest,LIS/lis-tempest,vedujoshi/tempest,openstack/tempest,zsoltdudas/lis-tempest,masayukig/tempest,LIS/lis-tempest,bigswitch/tempest