Modify check for volume-type-id to a get w/default

While implementing Fix for a previous bug to enable setting
volume type on a cloned volume and reading it's info we added
a vref['volume_type_id'], problem is that the _do_clone method
is shared for both clone and create from snapshot.

In the create from snapshot case however that key doesn't exist
in the vref (which is a snapshot object).

This change turns this into a key.get with a default return.

Fixes bug: 1192745

Change-Id: I1a6d38de614f54fcaa31ea7cc4250aacc85b0796
Merge "Modify check for volume-type-id to a get w/default"