Disassociate floating IPs from port on terminate

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
(cherry picked from commit f4b78c7f17e29448ed54b136eeb4ac700b324120)
Merge "Disassociate floating IPs from port on terminate" into milestone-proposed