LU-13056 utils: Default MGS device llog_print/llog_catlist

LU-13056 utils: Default MGS device llog_print/llog_catlist

Add the default device "MGS" for lctl llog_catlist, llog_print,
llog_check, llog_remove, llog_cancel and llog_info.

Example:
The two lines below are equivalent.

$ lctl --device MGS llog_catlist
$ lctl llog_catlist

Tests 123xx of conf_sanity have been modified to mix between the old
and new syntax.

lctl-llog_* man page have been modified to match the new syntax.

Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: If988dfc5d8171b2ca47bb68dd06f9d2953548cb2
Reviewed-on: https://review.whamcloud.com/38940
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>