Additional test cases for port forwardings API

This patch adds 2 additional test cases to API tests for
port forwardings:
1. Attempt to delete Floating IP used already by port forwarding -
   this should returns Conflict error,
2. Try to associate port forwarding to Neutron's port which is
   already associated with another Floating IP - this should raise
   Conflict error.

Depends-On: https://review.opendev.org/666235
Change-Id: I8b036068645f9d0277cde66b027a22273d9b1c41
1 file changed