Fix: libstonithd: add plug parameter besides port for RHCS-style fence-agents

According to:
https://github.com/ClusterLabs/fence-agents/commit/de490e059

, port parameter is massively deprecated in favor of plug:

<parameter name="plug" ... obsoletes="port">...
<parameter name="port" ... deprecated="1">...

The relatively new agent:
https://github.com/ClusterLabs/fence-agents/blob/master/agents/gce/fence_gce.py

doesn't even correctly handle port.

With this commit, plug parameter is added besides port with the same value.