Use AvailabilityZone instead of Availability (#246)

This commit updates the Block Storage v1 create request to use
AvailabilityZone instead of Availability.
diff --git a/openstack/blockstorage/v1/volumes/testing/fixtures.go b/openstack/blockstorage/v1/volumes/testing/fixtures.go
index c841676..306901b 100644
--- a/openstack/blockstorage/v1/volumes/testing/fixtures.go
+++ b/openstack/blockstorage/v1/volumes/testing/fixtures.go
@@ -82,7 +82,8 @@
 		th.TestJSONRequest(t, r, `
 {
     "volume": {
-        "size": 75
+        "size": 75,
+        "availability_zone": "us-east1"
     }
 }
       `)