LU-3006 utils: mount to pass/clear UPDATE flag

LU-3006 utils: mount to pass/clear UPDATE flag

this flag is maintained in mountdata (or fs attribute in case
of ZFS) and used to signal MGS parameters have been changed
and this need to be reflected in configuration profiles.

the flag is cleared before actual mount as the device can
be exclusively open by the filesystem preventing any updates.

the scope of the patch is limited to UPDATE flag, but later
we can try to use the approach to deal with WRITECONF/VIRGIN
flags as well.

test 73 added to conf-sanity to verify the case.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: Ic0b4eb6b86798450e4dd4201611e4f5a1c54ef40
Reviewed-on: http://review.whamcloud.com/5982
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>