LU-8727 mgs: remove skip records from config file

LU-8727 mgs: remove skip records from config file

Configuration logs are append-only files of limited size.  Over the
course of time the logs may grow over the limit size.  Usually,
configuration logs keep needless records marked as SKIP. The new lctl
command "clear_conf" is added to allow administartors to clear
configuration files by removing mentioned SKIP records. lctl man page
is updated.
conf-sanity test (for ldiskfs only) is added to test the new command.

Change-Id: I274cb48138c16e536cfca56836c3313e944eba56
Signed-off-by: Vladimir Saveliev <c17830@cray.com>
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Cray-bug-id: MRP-2091
Reviewed-by: Artem Blagodarenko <c17828@cray.com>
Reviewed-by: Alexey Leonidovich Lyashkov <c17817@cray.com>
Tested-by: Elena V. Gryaznova <c17455@cray.com>
Reviewed-on: https://review.whamcloud.com/23245
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>