Change keystone_default_role to _member_

Previously both Member & _member_ roles were created, which lead to
confision.
Member role is not created anymore as of [1].

[1] https://review.openstack.org/#q,If4ed1c2c72ca67acad7b70651e68ff7ce68157dd,n,z

Change-Id: I933236c2c257b65a1027da07a760c8b4d10618cc