Add volume_size_extend opt for volume tests

Volume tests had hardcoded value (1) in case they were creating
a second volume with a different size than the first one
(CONF.volume.volume_size). This is a problem for systems which
have a chunk size other than 1. The patch is adding a new opt
CONF.volume.volume_size_extend which allows customization of an
extended volume size.

Closes-Bug: 1917299
Change-Id: Ic8ae486224cd2a470f4f9bbad62d4d6715cc63ac
5 files changed