beats: resolve the output hosts once instead of in every template

[afeefghannam89]

The Beats templates worked out their own output hosts, nine times over, each with the same
elasticstack_full_stack branch. They now read beats_elasticsearch_hosts and
beats_logstash_hosts, set once in roles/beats/tasks/main.yml the way the kibana role does
it. A value the user sets wins. Otherwise, with elasticstack_full_stack true, the role fills
them from the inventory groups.

Two things that were not possible before:

• different hosts per output type, so Filebeat can go to Logstash while Metricbeat goes to
  Elasticsearch
• setting the hosts at all while elasticstack_full_stack is true

beats_target_hosts is removed. It fed both output types with one list and was ignored while
elasticstack_full_stack is true. With elasticstack_full_stack false, the two variables are
set directly, and the two Beats molecule scenarios now do that. Breaking, but the collection is
still 0.x.

The two set_fact tasks are tagged always, so every run has the two lists. The preflight
assert from #552 has the same tags as the configure tasks. It runs whenever a configuration gets
written and stops the run if a list is empty. A --tags certificates run writes no
configuration, so the assert does not run there. The rule behind this is now in
CONTRIBUTING.md.

Closes #557