Allow YAML params and serialize them to JSON

[andy-paine]

When using a task in Concourse, params can be provided as YAML objects and Concourse will just serialize the object as JSON and that is the value of the env var. Trying to write Ironbird tests that use YAML values as params fails as params is expected to be map[string]string. It would be good if it matched the behaviour of Concourse - i.e. params is map[string]interface{} which is then serialized