Use "is_neutron_security_groups" check

Module nova.network.security_group.openstack_driver contains
"is_neutron_security_groups" method, which checks that neutron is
turned on as security groups API.

This method can be reused in several places, so code duplication
will be reduced.

Change-Id: I7af1833772a4b6e5119c189554866c46466fcf06
Merge "Use "is_neutron_security_groups" check"