Handle port over-quota when allocating network for instance

This patch adds a check in the neutron API for a port create failure due
to over-quota in neutron and raises the new exception PortLimitExceeded.

Also moves the port-create block of code into it's own method to
try and clean up some of the large allocate_for_instance method.

Closes-Bug: #1207914
Related-Bug: #1209446

Change-Id: I4000c8ab550e032363f138a86e1b87f6ab2f5ff2
Merge "Handle port over-quota when allocating network for instance"