Fix: libstonithd: plug parameter of RHCS-style fence-agents is shown as non-required in the metadata

With this commit, the parameter plug of RHCS-style fence-agents is
fudged to be shown as required="0" in the metadata output by
stonith_admin --metadata or crm_resource --show-metadata. So that
users know they don't have to configure it. Pacemaker handles and passes
the parameter to the agent when executing it.