Adds a "node" field to Intel::Seen struture and intel.log.

The intel framework can now indicate which node discovered a
hit on an intel item through the new "node" field in the
Intel::Seen data structure.  On clusters, this field will
contain the name of the node where the hit was seen.
Fixes to plugin quick start doc.

BIT-1261 #close
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  Adds a "node" field to Intel::Seen struture and intel.log.