support a configurable volume wiping method

support a configurable volume wiping method

Add a clear_volume() method that is called from delete_method().
This is only implemented at present for the base LVM driver.
This new clear_volume() method now supports these config vars:

  volume_clear = none, zero, shred
  volume_clear_size = size_in_MiB

'zero' is the default method and unchanged from previously.
'none' is used if security isn't a concern, or there is
  independent volume scrubbing.
'shred' is for more security conscious situations where
an overwrite count is required. Currently this defaults to 3 passes.

size_in_MiB can be used to limit the cleared area to the first
part of the volume, which can greatly speed up the operation,
and can be useful with encrypted volumes for example where
overwritting the encryption keys at the start is sufficient.

Fixes bug: 1022511
Change-Id: I7312678ff5d66156578501483a1b74e2a69e19a0