Set Horizon default role to _member_

Keystone puppet module creates "Member" and "_member_" roles, which
creates confusion as to why both exist.  In a parallel change [1] we
modify keystone to only create the _member_ role.  Here we switch
Horizon to use that as the default role.

See also rhbz#984294 [2]

[1] https://review.openstack.org/#/c/60463/
[2] https://bugzilla.redhat.com/show_bug.cgi?id=984294

Change-Id: I329db398d0ec33eb3997167806717c45df7cec09
Merge "Set Horizon default role to _member_"