Add 'gather_facts: force' capability

This change adds the 'force' configuration value to the gather_flags
play attribute. With 'gather_facts: force' set, all hosts will have
facts gathered in accordance with the default fact gathering policy
(ie. smart, implicit, etc.).

This is useful in cases where --limit or --tags constrains the subset
of hosts that will be executed against, thereby limiting the number of
hosts that will be fact-gathered in the traditional sense.