Fix quantum nic allocation when only portid is specified

A simple omission at the end of allocate_for_instance() was preventing
the function from returning information about the newly-allocated port
if one was not actually allocated from a network (i.e. if it was
provided as input). This fixes that and adds a test which would have
caught it.

Fixes bug 1160560

(cherry-picked from 57689f26f37fe3de33a666a2d7d0731eae9c7ea9)

Change-Id: Ib0d7d91acfe5bd684cdfc072fe47c8115c2f91b3
Merge "Fix quantum nic allocation when only portid is specified" into milestone-proposed