Make allocate_for_instance() return only info about ports allocated

Previously, the compute manager was filtering the results based on the
port id that the user requested. However, if they do not pass one (which
is valid) then we can not do the filter and we fail. This makes the
allocation function do the filtering, which should not affect the normal
use case where it is only called once, since all the ports returned were
allocated at that time.

The manager filter behavior is thus removed and a test is added to
verify that the allocate method does the right thing the second time
around.

This fixes the fallout found after fixing the actual cause of bug 1131264

Change-Id: I8f3af96051268503596007491af365e8ce28f5b3
Merge "Make allocate_for_instance() return only info about ports allocated"