Combine all pulp django settings into single var

By creating a dictionary `pulp_settings` we can allow users to add or
update arbitrary values which will go into their local settings.py. This
also creates a clear scope separation between ansible variables and pulp
settings variables.

re #5031

[noissue]

README.md

@@ -17,8 +17,9 @@ Including an example of how to use your role (for instance, with variables passe
     ---
     - hosts: all
       vars:
-        pulp_secret_key: secret
         pulp_default_admin_password: password
+        pulp_settings:
+          secret_key: secret
         pulp_install_plugins:
           pulp-rpm: {}
       roles: