Fix local_settings.py on distributions

On RHEL, Fedora, Scientific Linux, CentOS, etc., the file
/etc/openstack-dashboard/local_settings.py (from Ubuntu)
is named /etc/openstack-dashboard/local_settings

Fixes bug 1179837

Change-Id: Ia91e6954e9589506823d9b5f8c44877b1d0d488f