Recently cliff introduced a sphinx extension directive named autoprogram-cliff
which genarates command-line help automatically. By using it, we no longer
need to write command-line help document separately.
Also fixes minor string substitution issue in a BGPVPN command.
Note that the new cliff release with the directive is not released yet,
but there are several number of OSC plugin commands proposed and
having a local copy would help such developers.
Change-Id: I6b1aee89f406ac449fbc43e210c4ca7ad901b19b
423890a495