Unify on _member_ instead of Member and _member_

Horizon is configured to use "_member_". However, the demo user uses
"Member". Since "_member_" is Keystone default, picking it as the
better value.

Change-Id: I281f22a26da7a15f0de907fd4fd14e04aa26758e
Merge "Unify on _member_ instead of Member and _member_"