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
Update log message for add_host_to_aggregate

When add host to aggregates with availability zone, if the host
is already in one availability zone, then the host cannot be added
to another availability zone.

When user do above operation, nova api will report error message,
but the error message does not include blank space between
<availability zone> and zone name, also the error message include
two periods.

The fix was adding blank space and removes one period for the message.

Change-Id: I7307288be194d3ce6de05b77433bbe2626ae46d9
Closes-bug: #1237303
(cherry picked from commit c2ae5fb5659faeadc6f4d8afe27eb68e3c0ade0c)
Merge "Fix for qemu-nbd hang" into stable/havana
Merge "Update log message for add_host_to_aggregate" into stable/havana