Move some content into the main Bro docs

Moved some content regarding how to setup a Bro cluster into the
main Bro docs (in a new section "Cluster Configuration").
Merge remote-tracking branch 'origin/master' into topic/dnthayer/ticket1186
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  Rename the broctl option pfringdnafirstappinstance
Merge remote-tracking branch 'origin/topic/dnthayer/ticket1186'

* origin/topic/dnthayer/ticket1186:
  Move some content into the main Bro docs

BIT-1186 #merged
Update broctl.rst with "make doc" (no other changes)