DataCollection and Rb::InstanceChooser fixes (more details inside)

* DataCollection.get_parameter_recursive will not join lists into strings
* Fixed Rb::InstanceChooser not to accept a string parameter, only lists
* Fixed a bug where Rb::InstanceChooser didn't handle pointers to lists
  properly
adding support for key:yaml.yaml to !yaml inside of templates
typo (+2 squashed commits)
Squashed commits:
[78d2df8] changed to requested syntax
[bce5c2b] Fails if the key is a non-string type
README fix: yaml datasource documentation was wrong