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
Service launcher method docstring corrected

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

Change-Id: Ic7934dd540a026141f525ea8ea6f976f959c7ea2
Merge "Service launcher method docstring corrected"
Merge "Set volume_dd_blocksize configurable on per-driver basis"