Query for port before calling l3plugin.disassociate_floatingips()

The call to l3plugin.disassociate_floatingips() trigggers several events
that could cause a timeout to occur trying to query the db for the port
therefore this patch changes the code to query first for the port.

Closes-bug: #1290478

Change-Id: I69fe9cff4b550240215329e1082a63cd11617faa
Merge "Query for port before calling l3plugin.disassociate_floatingips()"