Fix: fencer: add port or plug parameter according to metadata 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">...

With this commit, according to the metadata, if port parameter is not
supported, plug parameter is added if supported. By default, port
is added.