Fix broken solidfire create-snapshot

A previous change to enable secifying size for cloned volumes
created introduced a bug that breaks the ability to create-snapshots
when using the SolidFire driver
(CID: I5628c7fa922780d6b0601e2daa79310a61085edc).

The problem is that we use a shared method in the SF driver for both
create-clone and create-snapshot, the added change only considers the
clone case and does a get ref['size'] but in the case of snapshots
this needs to be ref['volume_size'].

Change-Id: I50603b3ac43f2c3c7e7811ec34de078a268519f7
Closes-Bug: 1234970
(cherry picked from commit 3cd27defb1c820ceadd8bbc6bfd5a405a7376cdb)