Handle NeutronClientException in secgroup create

When networking is provided by Neutron and a security group or a security group
rules quota is exceeded, we get a NeutronClientException with status_code 409
and message "NeutronClientException: Quota exceeded for resources"

This patch updates the neutron security group driver to handle this exception
and translate it to nova's SecurityGroupLimitExceeded

test_neutron_driver is also updated to provide unit tests for this patch

Change-Id: I0fa57b02694221507885ab4bfdb5805e936450e2
Fixes: bug #1209446
Merge "Handle NeutronClientException in secgroup create"