Add broctl option to set PF_RING cluster type

Added a new broctl option "PFRINGClusterType", which allows a user
to specify a PF_RING cluster type (the default is the same cluster
type used in previous versions of broctl).  The broctl pf_ring plugin
will use this information to set the correct environment variable
which is needed by the PF_RING-aware libpcap to set the cluster type.

Addresses BIT-1108
Change default PF_RING cluster type to 4-tuple
Add tests of the new PFRINGClusterType broctl option
Merge remote-tracking branch 'origin/topic/dnthayer/ticket1108'

* origin/topic/dnthayer/ticket1108:
  Add tests of the new PFRINGClusterType broctl option
  Change default PF_RING cluster type to 4-tuple
  Add broctl option to set PF_RING cluster type

BIT-1108 #merged