Support for password-protected Docker registries

Summary: Adds .dockercfg for authenticated access to Docker registries

Test Plan: Launch a stack with DockerCredentials specified and with MarathonDockerImage pulled from a private registry

Reviewers: merkkila

Reviewed By: merkkila

Subscribers: tberman, justin, stephan, serdar

Differential Revision: http://phabricator.thefactory.com/D2516