Consolidate the blockdev related filters

Seeing a "/usr/local/bin/nova-rootwrap: Unauthorized command" error
in the logs when  "blockdev --flushbufs" is executed because of the
existing blockdev in compute.filters. We need to merge both into a
single RegExpFilter

Change-Id: Ic323ff00e5c23786a6e376e67a4ad08f92708aef
Merge "Consolidate the blockdev related filters"