Sync OPENSTACK_KEYSTONE_DEFAULT_ROLE with keystone

For now, keystone default role is _member_, while horizon set
OPENSTACK_KEYSTONE_DEFAULT_ROLE to Member. It will really be user
friendly to modify horizon default value to _member_ to sync with
keystone's default setting.

Upstream-Bug: #1264228

Change-Id: I65e228a1f28a47908a7abd51a8fdf56b5180379d
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>