Require Boto v2.19.0 to work with IAM role

There is an issue with Boto v2.3.0 using IAM role to access S3.
As mentioned here (https://github.com/dotcloud/docker-registry/pull/72),
when 4 or more gunicorn worker process are trying
to fetch metadata for accessing S3, the requests are
throttled by Amazon and thus fail to launch the server.

Boto v2.10.0 and later introduces metadata_service_timeout &
metadata_service_num_attempts to resolve this issue.

This PR also comes with a Boto config file at /etc/boto.cfg to
increase default metadata timeout & attempts.