cascache.py: Add pull info detailing which remote is being pulled from

Work towards fixing issue #275

buildstream/_artifactcache/cascache.py

@@ -221,6 +221,8 @@ class CASCache(ArtifactCache):
             try:
                 remote.init()
 
+                element.info("Pulling {} <- {}".format(element._get_brief_display_key(), remote.spec.url))
+
                 request = buildstream_pb2.GetReferenceRequest()
                 request.key = ref
                 response = remote.ref_storage.GetReference(request)