feat(router): support for multiple routers

This commit adds functionality to the Makefile to launch multiple
routers, which is the first step towards a HA Deis installation.

Additional routers can later be added to an existing cluster by setting
DEIS_FIRST_ROUTER to start the router numbering at something other than 1.

Note that load balancing / DNS records must still be set up manually.

We also have the router publish its host:port to the namespace
/deis/routers/$HOST, as we have the possibility of multiple routers
in the cluster.

closes #922