build: exit if there is no Docker connection

When user starts Kolla on system without Docker daemon running huge
traceback is generated and then exit.

Issue is because KollaWorker raises exception but no one catches it
(then only call to it is in run_build() which assumes everything is ok).

Change-Id: I68ea3f48c75b51dbbab3accbc531d2f73428c48f

kolla/image/build.py

@@ -744,7 +744,9 @@ class KollaWorker(object):
                     or conf.save_dependency
                     or conf.list_images
                     or conf.list_dependencies):
-                raise e
+                LOG.error("Unable to connect to Docker, exiting")
+                LOG.info("Exception caught: {0}".format(e))
+                sys.exit(1)
 
     def _get_images_dir(self):
         possible_paths = (