Add ListImages() support and acceptance test.

This is needed to support a (currently broken) acceptance test for
creating a cloud server.  Without the ability to auto-detect an image, a
human operator is required to specify an image manually, which defeats
the value of an automated test.
diff --git a/interfaces.go b/interfaces.go
index 16c0589..ee02baf 100644
--- a/interfaces.go
+++ b/interfaces.go
@@ -18,6 +18,13 @@
 // CloudServersProvider instances encapsulate a Cloud Servers API, should one exist in the service catalog
 // for your provider.
 type CloudServersProvider interface {
+  // Servers
+
 	ListServers() ([]Server, error)
 	ServerById(id string) (*Server, error)
+
+  // Images
+
+  ListImages() ([]Image, error)
 }
+