Set volume_dd_blocksize configurable on per-driver basis

Allow volume_dd_blocksize config option to be configurable on a
per-driver basis.
This allows drivers used in multi-backend configurations
to define their own values. We port this to the LVM and block_device
drivers here but it can easily be ported to others as and when
needed.

DocImpact

partially implement bp: per-driver-config-for-utils-functions

Change-Id: Ice9e815f18e5ebf3727aab7d3a221acacb710cd4
Fix and enable gating on F401

Do not allow unused imports in modules, and remove those
that are currently unused.

Change-Id: I7600b1cce22eaef87633f98a89db2086a3ec2b55
Fix QoS information in initialize_connection() result

Currently the entire QoS information (if any) is included in the result of
initialize_connection() even if the consumer of the QoS is 'back-end'. Also
the format for QoS specs also is changed so that front-end (Nova) can
parse correctly. Add unit test to cover initialize_connection().

Closes-bug: 1259957

DocImpact

Change-Id: Ibc5e92cc1ddf6404e5b234ef524698feae282eec
Service launcher method docstring corrected

* Launcher run_server method parameter in docstring
  renamed to match actual argument name.

Change-Id: Ic7934dd540a026141f525ea8ea6f976f959c7ea2
Merge "Fix and enable gating on F401"
Merge "Service launcher method docstring corrected"
Merge "Set volume_dd_blocksize configurable on per-driver basis"
WIP Brick changes

Change-Id: Ie45889460dd4775a556d592161c98c24a56989c7