feat(contrib/ec2): add m3 instances to template

The current generation of EC2 instances (M3) is faster and cheaper
than the old M1 instances. It's a more reasonable default to use
m3.medium as the default instance size.

fixes #689
fix(contrib/ec2): bump size to m3.large

A 5GB root volume is too small. We might run into BTRFS issues
with low disk space.
fix(contrib/vagrant): fix Makefile loop

The Makefile condition was broken, and was always resetting to 3.
fix(controller): fix registry API calls

After upgrading the private registry to 0.6.8, there were some
changes to the API that were missed. For example, the docker version
is now mandatory[1].

If you have docker v0.10.0 or higher, there is also a checksum computation
check done on each push, so I hardcoded the user-agent to v0.9.0. This is
a stopgap until

a) we update the API calls to v0.10.0
b) we replace this component with a docker-registry python package[2]

[1]: https://github.com/dotcloud/docker-registry/commit/64f4198c9041c25ac416b8ebba5c57304315156c
[2]: https://github.com/dotcloud/docker-registry/pull/297

fixes #779
Merge pull request #782 from opdemand/fix-registry-errors

fix(controller): fix registry API calls
Merge pull request #768 from opdemand/add-m3-instances

feat(contrib/ec2): add m3 instances to template
Merge pull request #780 from opdemand/fix_makefile_loop

fix(contrib/vagrant): fix Makefile loop
refactor(contrib/coreos): update override-plugin and user-data

This commit updates the override-plugin and user-data to better
reflect upstream, and to enable both the 1-node and n-node Vagrantfiles
to use the same user-data. This is easier to maintain, and allows
us to pull in upstream changes with greater ease.

closes #760