Use `member` instead of `Member`

Keystone now provides a set of default roles in addition to `admin`
by default [0]. This is done during the `keystone-manage bootstrap`
process.

This change aligns the `Member` role override from devstack with the
`member` role provided from keystone.

[0] https://review.openstack.org/#/c/572243/

Change-Id: I3da3530aa73a8a1500116bcefdcba7b947d5e05e
Closes-Bug: 1777359
diff --git a/lib/horizon b/lib/horizon
index fab41bb..293a627 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -87,7 +87,7 @@
     _horizon_config_set $local_settings "" WEBROOT \"$HORIZON_APACHE_ROOT/\"
 
     _horizon_config_set $local_settings "" COMPRESS_OFFLINE True
-    _horizon_config_set $local_settings "" OPENSTACK_KEYSTONE_DEFAULT_ROLE \"Member\"
+    _horizon_config_set $local_settings "" OPENSTACK_KEYSTONE_DEFAULT_ROLE \"member\"
 
     _horizon_config_set $local_settings "" OPENSTACK_HOST \"${KEYSTONE_SERVICE_HOST}\"