Skip setting volume_size if not given

When the block-device parameters skip volume_size,
don't set it. Setting to an empty volume_size
would be invalid as it has to be an integer,
and Nova API will reject the request if api validation
is implemented. (proposed e.g. at
https://review.openstack.org/#/c/34749/)

Fixes bug LP #1199539

Change-Id: I7ab518886abf8d449caf1c70563a79a990d7765e
Merge "Skip setting volume_size if not given"