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
docs(*): add bare-metal guide

Adds some light documentation around bare metal provisioning.

closes #730
Merge pull request #979 from deis/baremetal_docs

docs(*): add bare-metal guide
Merge pull request #969 from deis/multiple_routers

feat(router): support for multiple routers
docs(bare-metal): filled in bare metal section in provision doc

We need a docs.deis.io anchor for bare metal to reference from deis.io.
Merge pull request #982 from deis/update-provision-doc

docs(bare-metal): filled in bare metal section in provision doc