Wait for the router port activation before deletion

Same as in [1], it is needed to use the same active wait for the
testcases in ``RoutersNegativeTest``. This patch moves the router
interface deletion method to a common place.

[1]https://review.opendev.org/c/openstack/tempest/+/931765

Related-Bug: #2083287
Change-Id: I224d885b6f56f9a1bc7a317995993be242feba23
3 files changed