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
Rename the broctl option pfringdnafirstappinstance

Renamed pfringdnafirstappinstance to pfringfirstappinstance (i.e.,
removed the "dna" in the name) because the name is quite long and
this option is also used with ZC (and DNA is supposedly going away
sometime in the future).
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