Make availability_zone optional in create for aggregates

As the novaclient's help message, we can create aggregate without
availability_zone. But the api didn't allow it, so this patch make
it optional.

Closes-bug: 1252177

DocImpact

Change-Id: I64629d71648900bf5be554d9afa4ae47d6b651d0
Merge "Make availability_zone optional in create for aggregates"