Vagrantfile: Use default config for Virtualbox provider.

* config.vm.box and config.vm.box_url for the default config are the
  same as the virtual box provider.  Remove the overrides.
Vagrantfile: Use default box name for all providers.

* Use the BOX_NAME constant for all providers instead of using a
  different box name for different providers.  This can still easily be
  overriden through setting the BOX_NAME environment variable.
Vagrantfile: Use the same ssh private key path 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).
Vagrantfile: Allow AWS instance type to be specified from the environment.

* 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.
Vagrantfile: Use the same environment variables as ec2-api-tools.

* 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 further environment configuration.
Documentation: Update the AWS vagrant section to reflect Vagrantfile changes.

* Update the AWS vagrant section to reflect changes to the Vagrantfile.
   - Add a step about adding a vagrant box for the aws provider.
   - Update the environment variable names and descriptions.
Vagrantfile: Specify 12.04lts for each region through aws.region_config.

* 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.
Vagrantfile: Rewrite the provisioning code.

* 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.