Use get('field', default) to assign downburst values for vps.  Also look
in ~/.VM_TYPE.downburst.yaml (~/.ubuntu.downburst.yaml for example) to
set specific downburst values, if desired.

Fixes: #4592
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Reviewed by: Dan Mick <dan.mick@inktank.com>