Unhardcode glance user/group uid/gid values
Warn to not change glance uid/gid values after user is created

Change-Id: Iff3e6490c14134c3cda12035578dd153b1621aea
diff --git a/README.rst b/README.rst
index c791b9a..04f22eb 100644
--- a/README.rst
+++ b/README.rst
@@ -17,6 +17,8 @@
         enabled: true
         version: juno
         workers: 8
+        glance_uid: 302
+        glance_gid: 302
         policy:
           publicize_image:
             - "role:admin"