Add support for deploying NixOS containers

You can now deploy a NixOS container on localhost using a spec like:

  foo = { deployment.targetEnv = "container"; };
Merge branch 'master' into containers
Add convenience command for starting a development shell
Improve container creation
Support starting and stopping containers
Remove hard-coded path
Allow building hetzner-bootstrap on any host
add deployment.targetPort option
GCE backend: initial implementation
GCE backend: simplify root disk creation for most use cases, make it harder for complex cases
GCE: Disks: implement error checking, handling, and resolution.
Merge pull request #190 from shlevy/hetzner-from-osx

Allow building hetzner-bootstrap on any host
Update examples
Merge branch 'master' into containers
Merge pull request #191 from iElectric/targetPort

add deployment.targetPort option
Per Eelco's suggestion, use first configured SSH port as default for deployment.targetPort.
Update nix-homepage example

In particular, open the firewall.
Don't run dhcpcd in containers
Support Amazon EBS SSD disks (http://aws.amazon.com/blogs/aws/new-ssd-backed-elastic-block-storage/).
GCE: Network and Firewall: complete --check implementation