api.play_on: Clarify the use of `gather_facts`

Situation before this patch.

The gather_facts option was supposed to control whether the facts are gathered prior to the execution of the modules. It could take two values:

• False -> facts was not gathered at all
• True -> facts for the hosts maching the provided pattern_hosts were gathered. Note that this contradicted the docstring who stated that the facts were supposed to be gathered on all hosts

Situation with this patch.

gather_fatcs is now an Union of boolean and string:

• True -> for the hosts maching the provided pattern_hosts were gathered
• False -> no fact will be gatherd
• pattern_hosts: str -> facts will be gathered for hosts that match pattern.