Explicitly set auth_strategy

On centos systems, /usr/share/neutron/neutron-dist.conf sets:

auth_strategy = noop

In order to set this correctly back to the upstream default of keystone we
need to explicitly set it in neutron.conf to override the
setting in the dist file

Change-Id: I9b5d7b629a97e6f8ec758fbea759d1ddf3fb039a
Implements: blueprint centos-rhel-for-network
Merge "Explicitly set auth_strategy"