Update devstack/README.rst
Update devstack/README.rst

       * Horizon is enabled by default, so there is no need to enable it.
       * The Gnocchi plugin should be enabled
       * The CloudKitty services need to be enabled

Change-Id: I2b0bac8c8c62479a4701f66efc2703e12ecbaaa3

openstack/cloudkitty,stackforge/cloudkitty,openstack/cloudkitty,stackforge/cloudkitty