LU-5657 doc: allow the use of rst2man to build man pages

LU-5657 doc: allow the use of rst2man to build man pages

The man page sources can now be written in reStructuredText (rst), and
the man pages will automatically be generated with rst2man.

Added a build dependency on rst2man and the package python-docutils.

Converted lustreapi.7 to ReST to validate the solution.

Change-Id: I69e9892a238a002eb86769ed65b758cba55543bb
Signed-off-by: frank zago <fzago@cray.com>
Reviewed-on: http://review.whamcloud.com/12040
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>