LU-6356 mgs: fix security flavor setting for connection to mgs

LU-6356 mgs: fix security flavor setting for connection to mgs

Security flavor setting for connections to MGS is done with special
target '_mgs'. If set, only allow connections to MGS with the given
flavor. If not set, all flavors must be allowed.

Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Change-Id: Id47aae25f6d5591870656d9926a64e52ab789c56
Reviewed-on: http://review.whamcloud.com/14349
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Jeremy Filizetti <jeremy.filizetti@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>