LU-8900 snapshot: fork/erase configuration

LU-8900 snapshot: fork/erase configuration

Introuduce new lctl command to fork/erase the configuration for
the specified system. It is used for mount the snapshot together
with the snapshot's original system MGS. Usage:

lctl fork_lcfg <fsname> <newname>
lctl erase_lcfg <fsname>

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I55d420b54215b09d42b2496b5c1271211c1fb5d7
Reviewed-on: https://review.whamcloud.com/24266
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>