When building a complex stack, instance and volume are both
necessary. But the support of AZ in Nova and Cinder are slightly
different which is causing troubles to build the Heat template.
Without setting, the property availability_zone of volume always
got "", an empty string, instead of None. As a result, Cinder will
return 400 BadRequest error. This patch will turn the empty
string into None, to make sure Cinder can correctly handle the
scenario when availability_zone of volume is not set.
Task: 38853
Story: 2007330
Change-Id: Ib5bddf12ca63849a030d1d579d0a2e853e8b848a
50d72b8c0b