Start using 'cliff.sphinxext'

'cliff', the command line library used by 'osc_lib' (and, thus,
'python-openstackclient') recently gained a Sphinx extension to
automatically document cliff commands. This allows us to use the
documentation we already have in code instead of duplicating it in the
documentation.

Introduce the use of this, starting with the 'server' commands. This
requires extending the descriptions for two commands to ensure no
information is lost.

Change-Id: If701af8d5a3f78f4b173ceb476dd0c163be4b6ca

Author: stephenfin