WIP
Merge branch 'configuration-defaults'

Multiple plugins can now register default values for a single
configuraiton option.  When the configuration object is built,
they are each processed until a non-nil value is returned from
one of the defaults.
Added Aws::InstanceProfileCredentials.

This class attempts to load credentials from the EC2 instance
metadata service if it is available.  If the metadata service is
not running locally, then an empty set of credentials is returned.

These credentials will also automatically refresh if used within
5 minutes or later of expiration.
Added the InstanceProfileCredentials plugin to each service.

This plugin attempts to load credentials from the EC2 metadata
service.  This happens only if static credentials are not
present.