Add default_availability_zone configuration option to cinder

Add default_availability_zone configuration option to cinder

This option is used to specify the default AZ for the whole service. So when
creating a new volume this value is used to specify the AZ of a volume.

Previously we were overloading the use of the storage_availability_zone option
which is also used to specify what AZ a service belongs to. We still fail back
on the storage_availability_zone when creating a new volume if the
default_availability_zone is not set.

Fixes bug: 1221260

Change-Id: Iec381f50c9aeb4a0abbcaa1d7b0107de09a73544