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' with less environment configuration
  than 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.
- Update the AWS install documentation to reflect the changes.