Bugfix - floating IPs were left associated after VM
was terminated. Now call disassociate_floatingips
within delete_port as in other networking plugins.
Add L3NatDBIntTestCase suite to cover the
floating IP disassociation case, and fix all failing
tests from that suite.
Change-Id: I856c46631e495d513065fc9e987898408441a21e
Closes-Bug: #1231913