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
Update SDK tools to version 24.0.0

Update SDK tools version to 24
Update SDK tools checksum (using sha256sum)
Remove woraround to update tools using previous version.

This update fixes all the issues about SDK tools version 23.0.5 required by Android Lollipop and new updates.

Google released a new version of SDK tools, further information here:
https://developer.android.com/sdk/index.html#Other

Download:
http://dl.google.com/android/android-sdk_r24-linux.tgz
Merge pull request #407 from ardock/patch-1

Update SDK tools to version 24.0.0