Initial commit for setting up Docker host for Travis CI

Adding 'travis_docker' to run_list should do the job.

WARNING: This is completely untested
Make travis_docker cookbook self-contained
Move travis_docker cookbook to worker_host
Add travis_docker metadata
Prefer remote_file Chef resource to 'wget'
Prefer apt package to another cookbook
Fix package name
Updates to get the Docker worker to build
travis-worker: fail if docker pull fails
Add collectd recipe for Docker workers
rename travis_worker_collectd::docker to ::ec2-docker

This fits more with our other recipe names.
Updating travis-worker to pull `latest` instead of `dev` image tag
Merge pull request #399 from travis-ci/db-tw-latest-tag

Updating travis-worker to pull `latest` instead of `dev` image tag
docker-collectd: report disk usage in percentages

We're deploying this to machines with varying disk sizes, so tracking the
percentages makes it easier to set up alerts.
Merge pull request #400 from travis-ci/hh-collectd-percentage

docker-collectd: report disk usage in percentages
Update RVM to 1.26.2
Add netcat

Fixes https://github.com/travis-ci/travis-ci/issues/2945
Update RVM to 1.26.3
Only track network traffic for some interfaces on EC2

It seems there is a randomly named network interface, which causes a lot of
different metrics to be created.
PostgreSQL: fix a bug in init.d script

Without this change the return code of 'status' is wrong.