Change value of SECRET_KEY

Currently SECRET_KEY points to
{ install_lib_dir }}/openstack_dashboard/local/.secret_key_store
however this directory is not writable by www-data:www-data and
attempting to load horizon results in a LockFailed exception being
raised.  Rather than making
{ install_lib_dir }}/openstack_dashboard/local/ writable by
www-data:www-data, we create /var/lib/horizon and point SECRET_KEY to
/var/lib/horizon/.secret_key_stone.