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"
Merge commit 'refs/changes/53/58753/9' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch
Merge commit 'refs/changes/54/58754/11' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch
Merge commit 'refs/changes/55/58755/10' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch
Merge commit 'refs/changes/61/58861/10' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch
Merge commit 'refs/changes/53/60253/7' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch

Conflicts:
	nova/virt/xenapi/vm_utils.py
Merge commit 'refs/changes/08/60808/6' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch
Merge commit 'refs/changes/78/63778/1' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch
Merge commit 'refs/changes/38/66338/7' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch
Merge commit 'refs/changes/97/66597/3' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch