Fix for qemu-nbd hang

NBD device once used seem to run into intermittent trouble when
used with mount repeatedly. Adding code to explicitly flush the
device buffers using 'blockdev --flushbufs'.

NOTE(dmllr): Also merged regression fix:

    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

(cherry-picked from dd3f96e91581f465a52d10a212eb51b147dc38b5)
(cherry-picked from 31b2791e6be1768c410de5fa32283fcb637bda57)

Closes-Bug: #973413
Partial-Bug: #1254890
Change-Id: I2b7053b9a069d6e82f6f6baf9ad480efa4388d91
Merge "Fix for qemu-nbd hang" into stable/havana