Using the correct nova method to get the image service

Fixes OS-102

Nova changed the location of the method to retrieve the default
image service. Instead of it being in the __init__.py of the image
package, it has been place directly in the glance module. This
applies this change to the extension.

Change-Id: I700545959fc07d456e7e544f821312bcca94480b
Merge "Using the correct nova method to get the image service" into stable/folsom