EC2 provider configuration
--------------------------

The EC2 provider accepts a number of configuration options, that can
be specified in the ``environments.yaml`` file under an ec2 provider section.

    access-key:
        The AWS access key to utilize for calls to the AWS APIs.

    secret-key:
        The AWS secret key to utilize for calls to the AWS APIs.

    ec2-uri:
        The EC2 api endpoint URI, by default points to `ec2.amazonaws.com`

    region:
        The EC2 region, by default points to `us-east-1`. If 'ec2-uri' is
        specified, it will take precedence.

    s3-uri:
        The S3 api endpoint URI, by default points to `s3.amazonaws.com`

    control-bucket:
        An S3 bucket unique to the environment, where some runtime metadata and
        formulas are stored.

    ensemble-branch:
        Allow a ensemble bzr branch to be utilized for all machines within an
        environment.

    default-instance-type:
        The instance type to be used for machines launched within the ensemble
        environment.

    default-image-id:
        The default amazon machine image to utilize for machines in the
        ensemble environment. If not specified the default image id varies by
        region.


Additional configuration options, not specific to EC2:

    authorized-keys-path:
        The path to a public key to place onto launched machines. If no value
        is provided for either this or ``authorized-keys`` then a search is
        made for some default public keys "id_dsa.pub", "id_rsa.pub",
        "identity.pub". If none of those exist, then a LookupError error is
        raised when launching a machine.

    authorized-keys:
        The full content of a public key to utilize on launched machines.