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
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.
Point collectd df at "/", "/mnt", and "/data/travis" on travis-worker hosts
Merge pull request #406 from travis-ci/db-travis-worker-collectd-df

Point collectd df at "/", "/mnt", and "/data/travis" on ec2 travis-worker
Cutoff for December 2014 updates
Android: update 'platform-tools' before 'tools'

This is a bugfix for travis-ci/travis-cookbooks#403.
Otherwise, the latest SDK tools cannot be installed:

```
Installing Archives:
  Preparing to install archives
  Skipping 'Android SDK Tools, revision 23.0.5'; it depends on 'Android
  SDK Platform-tools, revision 21' which was not installed.
  Done. Nothing was installed.
```

References and Credits:
 - https://github.com/travis-ci/travis-ci/issues/2931
 - Special thanks to @ardock for stressing this problem out
(https://github.com/travis-ci/travis-ci/issues/2848#issuecomment-64977198)