Adding new configuration parameter 'storwize_svc_retain_aux_volume'
to storwize cinder driver configuration guide which is an optional
paramter to retain auxiliary volume in a mirror relationship on delete
of the primary volume or moving the primary volume to non-mirror
Change-Id: Ic2e696605a5f2176fdd8a88e8ec76190a45a27c1
3d55a22fe7