Make the fake metadata server more general. (#1619)
Make the fake metadata server more general. (#1619)

Previously, the server listened on a hard-coded IP address, which
made changing that IP address impossible.

With this change, the server instead listens on a fixed hostname,
so that hostname can be bound to different IP addresses.
craigcitro/datalab,googledatalab/datalab,parthea/datalab,parthea/datalab,googledatalab/datalab,craigcitro/datalab,parthea/datalab,craigcitro/datalab,googledatalab/datalab,yebrahim/datalab,parthea/datalab,yebrahim/datalab,yebrahim/datalab,parthea/datalab,parthea/datalab,googledatalab/datalab,yebrahim/datalab,yebrahim/datalab,craigcitro/datalab,craigcitro/datalab,googledatalab/datalab,googledatalab/datalab