Fix port creation issue appeared with postgresql backend

IPAllocationPool has relation to IPAvailabilityRange which is setup to
load eagerly. Eager loading is implemented with left outer join which is
incompatible with with_lockmode('update') on postgresql.
The fix redefines eager loading with options(joinedload) making it use
inner join.

fixes bug 1215350

Change-Id: I148d37f2a1c2a340327d36c240eb8173aee9219a
Merge "Fix port creation issue appeared with postgresql backend"