Add support for setting default_availability_zone (bnc#909490)

https://bugzilla.suse.com/show_bug.cgi?id=909490
(cherry picked from commit cfa707645a78cd9572a8b3b8bc9bd45ade261860)
Merge pull request #219 from vuntz/default-az-stoney

Add support for setting default_availability_zone (bnc#909490)