nova-dhcpbridge should require the FLAGFILE is set

We always launch dnsmasq with FLAGFILE set, so something is very broken
if FLAGFILE isn't set when nova-dhcpbridge is launched.

Also, the fallback to CONF.dhcpbridge_flagfile is a little strange - at
this point, we haven't parsed any config files so the only possible
value of this config option is its default value.

There's no real need to gracefully handle this condition either - a
KeyError is a straightforward enough error.

Change-Id: I734d7b739f17c8a9ab48d8ba13baacff3f00e1c4

bin/nova-dhcpbridge

@@ -95,8 +95,8 @@ def init_leases(network_id):
 
 def main():
     """Parse environment and arguments and call the approproate action."""
-    flagfile = os.environ.get('FLAGFILE', CONF.dhcpbridge_flagfile)
-    argv = config.parse_args(sys.argv, default_config_files=[flagfile])
+    argv = config.parse_args(sys.argv,
+                             default_config_files=[os.environ['FLAGFILE']])
     logging.setup("nova")
 
     if int(os.environ.get('TESTING', '0')):