ovirt_remove_stale_lun: Retry "multipath -f" while removing the LUNs

[nijinashok]

There are possiblities that the vdsm might be holding the LUN
temporarily while we run "multipath -f". In these cases, the flushing
fails with error "map in use". Retry multipath -f (6 * 5 seconds) to
workaround this race condition.

RHBZ: https://bugzilla.redhat.com/2023224

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[mnecas]

Please review @vjuranek

[mnecas]

ci add to whitelist

[vjuranek]

LGTM, thanks @nijinashok !