Add support for explicitly specifying a "drop" action when adding a flow.

Add support for explicitly specifying a "drop" action when adding a flow.

When adding a flow entry with dpctl, the user specifies a set of actions
to execute when a match occurs.  If none are specified, then the packet
is implicitly dropped by OpenFlow.  When dumping these flows, the output
of dpctl shows an action of "drop".  However, dpctl did not support
adding a flow with an explicit "drop" action.  This fixes that lack of
symmetry.