Vagrantfile updates.

- Remove the overrides config.vm.box and config.vm.box_url and use
  the same values for all providers.
- Use the same private key path for all providers.  It is still possible
  to set a different private key path through the environment variable
  SSH_PRIVKEY_PATH if desired (your AWS key may be different from your
  Virtualbox key).
- Allow the environment variable AWS_INSTANCE_TYPE to specify the
  instance type of instead of hard coding the AWS instance type as
  't1.micro'.  't1.micro' is still the default if unspecified.
- Use the same environment variables for keys as the Amazon provided EC2
  API tools.  This allows people who already have the EC2 tools set up
  correctly to use 'vagrant up' without less environment configuration
  than before.
- Use the Canonical published 12.04lts AMI for each region.  This is
  done through 'aws.region_config'.  Allow the AMI to be overriden
  through the environment variable 'AWS_AMI' just as before.
- Rewrite the provisioning code.  The goal is to be idempotent and to
  correctly install docker for all providers instead of just virtualbox.
  It will conditionally install the virtualbox guest additions if
  virtualbox is the provider.  The guest additions must be installed while
  the 3.8 kernel is running.  This means a reboot before the installation
  of the guest additions, just as the old code had done.
- Update the AWS install documentation to reflect the changes.