LU-6437 doc: enable lustre to be built without manpages

LU-6437 doc: enable lustre to be built without manpages

Add configure option --disable-manpages, to prevent the building and
packaging of the manpages. This can be used for an embedded
environment, or when manpages can't be build because python-docutils
(rst2man) isn't available.

Do not distribute (with make dist) manpages generated from ReST, but
distribute the ReST sources.

Signed-off-by: frank zago <fzago@cray.com>
Change-Id: I47405d466ac35eb9d295c5a6dc0bdeb0d4d75b9e
Reviewed-on: http://review.whamcloud.com/14382
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>