Use bare container format by default

Set container_format to bare during libvirt snapshot, when VM image in
glance was deleted. Currently, if VM image in glance was already deleted
before snapshot, nova will attempt to create snapshot image with
container_format: None. This cause glance to return error on attempt to
upload snapshot. According to glance docs container_format is not used
anywhere in glance or nova explicitly and it is safe to set it to bare,
when you are unsure which container_format you need to use.
Current snapshot logic sets snapshot disk_format to currently used
image_format in absence of base image in glance.

This resolves bug 921774 without need for snapshot mechanism redesign.

Change-Id: I7beea35120aaeac0837daecdf58f38f62e24454c

nova/virt/libvirt/driver.py

@@ -784,8 +784,7 @@ def snapshot(self, context, instance, image_href):
         else:
             metadata['disk_format'] = image_format
 
-        if 'container_format' in base:
-            metadata['container_format'] = base['container_format']
+        metadata['container_format'] = base.get('container_format', 'bare')
 
         # Find the disk
         xml_desc = virt_dom.XMLDesc(0)