Determine security group API dynamically

This commit removes enable_security_group from local_settings.py
and determines which security group API should be used (nova or
neutron).

Closes-Bug: #1227804

As described in bug 1203413, there is a case where Nova security group
with Neutron driver causes a problem. The type of 'name' attribute in
add_security_group_to_instance and remove_security_group_from_instance
depends on the backend, integer for nova security group driver and
UUID for quantum security group driver to make it work as expected.

enable_security_group config parameter produces a situation where
Nova security group with Neutron driver. We can avoid this situation
by removing this parameter when using Horizon.

Change-Id: I713c6ad166e142929f0a708e93a8fedb0de48640
Merge "Determine security group API dynamically"