Fix fact gathering with --limit

Prior to this change, when the --limit argument is used, each host in the
limit gathers facts for every other host. This is clearly unnecessary, and
can result in up to (N-1)^2 fact gathers.

This change gathers facts for each host only once. Hosts that are not in
the limit are divided between those that are in the limit, and facts are
gathered via delegation.

This change also factors out the fact gathering logic into a separate
playbook that is imported where necessary.

Change-Id: I923df5af41a7f1b7b0142d0da185a9a0979be543