fence_scsi: Fixes for auto-detecting devices for shared VG

[arslan9292]

In the earlier version, the shared devices can be auto-populated using the 'c' flag in 'vgs' command output when shared VG is configured with clvm. With the release of lvmlockd, the shared VGs has 's' flag under 'Attr' section.

So as to make the auto-detection feature working for lvmlockd, this update will allow the fence/unfence operation succeed even when the 'devices' attribute is left blank or undefined.

[knet-ci-bot]

Can one of the admins verify this patch?

[oalbrigt]

ok to test

[nrwahl2]

The following aren't necessary for this PR. They're just things that I noticed.

• It looks like the fence_scsi metadata (help output) doesn't even advertise that you can leave devices blank to get a dynamic list based on the 'c' bit... it's probably a good idea to add a note, unless we plan to deprecate that feature.
• The metadata for fence_mpath says the devices have to be a comma-separated list, but that's not true -- they can be space-separated since 2d0057d. The fence_mpath metadata should be updated.
• 2d0057d didn't add support for space-separated devices in fence_scsi. We should probably add support for that syntax to fence_scsi if we support it in fence_mpath.

[oalbrigt]

Thanks.