We need to define _glance_available_stores outside glance role to
use it in haproxy service definition.
It's a good idea to make `_glance_available_stores` public by moving it
out of role variables to role defaults beforehand.
Change-Id: Ieb10a0e5c9faf72c6bea4c45f7e216469971a1f3
fafd49959e