fix(scheduler): announce proper address

Before, the announce container relied on `getent` to retrieve the
hostname. Some hosts do not have `getent hosts deis` properly
routed, which would make the router crash and burn because it would
try to route traffic to `:1234` instead of `10.0.2.14:1234`.
Instead, the announce container now retrieves the host's IP address
from /etc/environment. This is the public IP address, which is
written to /etc/environment.

See https://github.com/deis/deis/blob/master/contrib/coreos/override-plugin.rb#L87-L92

fixes #797
Merge pull request #814 from deis/fix-announce

fix(scheduler): announce proper address