fix error when not specifying DockerCredentials parameter
fix error when not specifying DockerCredentials parameter

The previous fix for this only modified the default value in the sub-templates, but I was still getting an error when spinning up the entire stack using the main template, since the value is passed into the sub-templates, rather than their default used.
gengmao/cloudformation-mesos,zooniverse/cloudformation-mesos,zooniverse/cloudformation-mesos