LU-17409 scripts: correct ldev MGS handling

LU-17409 scripts: correct ldev MGS handling

ldev was incorrectly parsing the line specifying the hosts that can
run the MGS, when of the form

    gopher1  gopher2  MGS  gopher1/mgs

as it assumed every target specified included a filesystem name,
like 'lustre3-MDT0000'.

This corrects that, assuming that an MGS may not be related to a
specific file system.

When such an input line is found, assume that MGS is used by any file
systems included in the ldev.conf.  When user includes option '-F
<fsname>' as well as '-R MGS', include that MGS, in the targets
reported.

Lustre-change: https://review.whamcloud.com/53619
Lustre-commit: bb6a2d2e80f04645b488ecca6ba14cb628e3eeb3

Test-Parameters: trivial
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: Ifab5db1dfb094755e29747ec6b90d1566b16c18c
Reviewed-by: Cameron Harr <charr@llnl.gov>
Reviewed-by: Eric Carbonneau <carbonneau1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54667
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>