Make dd block size user configurable

This patch introduces a new configuration option volume_dd_blocksize
to change the bs parameter passed to the dd command when creating or
deleting volumes. In certain scenarios changing the block size might
result in better performance.

The default value is still 1M, which was used before.

Change-Id: I0398436ab3ecca51f6121e8f521af8b2dc60acd3
Fixes: bug #1157417
Merge "Make dd block size user configurable"