Catch CannotResizeDisk exception when resize to zero disk

When old flavor's root_gb is not equal 0 and new flavor's root_gb is 0,
the resize() in nova.compute.api will raise CannotResizeDisk.

Move up the new_instance_type check before using.

Closes-Bug: #1340159

Change-Id: I4abf93530cf919af50a88d6049019fb745547257
Merge "Catch CannotResizeDisk exception when resize to zero disk"