Call _destroy_metadata_proxy from _destroy_router_namespaces

Refactor _spawn/destroy_metadata_proxy so that it can be called
with only the namespace and the router_id.

Change-Id: Id1c33b22c7c3bd35c54a7c9ad419831bfed8746b
Closes-Bug: #1252856
(cherry picked from commit 07d597079781967f5a149f1812ddca3897fa49d9)
Related-Bug: #1175695
Partial-rally-bug: DE745
Upstream-review: https://review.openstack.org/#/c/84418/