apcups: allow to use plugin without explicit configuration

apcups: allow to use plugin without explicit configuration
Previously the apcups plugin could not be used without defining
an explicit configuration.

While the apcups plugin did provide a localhost fallback, it
did not however provide a 3551 port fallback in practice,
eventhough a constant for that purpose was already present.

Thanks to Pascal de Bruijn for report.

Closes: #2347