Minor fix in Keystone charm config

Fix the default value for the config ldap_user_enabled_default in the
Keystone charm, used by the charmed installation of OSM

Change-Id: I0272c1c7218d999e6d1ad7755883961ace997a81
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/installers/charm/keystone/config.yaml b/installers/charm/keystone/config.yaml
index 2a54a3d..2758d1c 100644
--- a/installers/charm/keystone/config.yaml
+++ b/installers/charm/keystone/config.yaml
@@ -191,7 +191,7 @@
       enablement. However, some schemas might use an integer value in an
       attribute. In this situation, set user_enabled_default to the integer
       value that represents a user being enabled.
-    default: true
+    default: "true"
   ldap_user_enabled_invert:
     type: boolean
     description: |