Don't return empty string as valid az for volumes

Since OpenStack Liberty, Nova fails if create_vm
contains an element 'availablity_zone' in the body
with an empty string as value.

Fixes #54
[#135269529](https://www.pivotaltracker.com/story/show/135269529)

Signed-off-by: Tom Kiemes <tom.kiemes@sap.com>

src/bosh_openstack_cpi/lib/cloud/openstack/availability_zone_provider.rb

@@ -12,7 +12,7 @@ def select(volume_ids, resource_pool_az)
         fog_volume_map = @openstack.volume.volumes
         volumes = volume_ids.map { |vid| with_openstack { fog_volume_map.get(vid) } }
         ensure_same_availability_zone(volumes, resource_pool_az)
-        volumes.first.availability_zone
+        ignore_empty_string(volumes.first.availability_zone)
       else
         resource_pool_az
       end
@@ -24,6 +24,12 @@ def constrain_to_server_availability_zone?
 
     private
 
+    def ignore_empty_string(string)
+      if string
+        string unless string.empty?
+      end
+    end
+
     ##
     # Ensure all supplied availability zones are the same
     #

src/bosh_openstack_cpi/spec/unit/availability_zone_provider_spec.rb

@@ -22,6 +22,26 @@
     let(:ignore_server_az) { false }
 
     describe 'when the volume IDs are present' do
+      describe 'when az of volume is empty string' do
+        before do
+          allow(bar_volume).to receive(:availability_zone).and_return('')
+        end
+
+        it 'returns nil' do
+          expect(az_provider.select(['bar_id'], nil)).to be_nil
+        end
+      end
+
+      describe 'when az of volume is nil' do
+        before do
+          allow(bar_volume).to receive(:availability_zone).and_return(nil)
+        end
+
+        it 'returns nil' do
+          expect(az_provider.select(['bar_id'], nil)).to be_nil
+        end
+      end
+
       describe 'when the volumes and resource pool are all from the same availability zone' do
         before do
           expect(bar_volume).to receive(:availability_zone).and_return('west_az')