Werk #11880: ups_capacity: Bring back UPS 'capacity' metric

In versions 1.6.0p12 to 1.6.0p19 the 'capacity' metric was not produced if the correspinding devices were on main power supply. If it was produced, it was the canonical unit sections, which is desirable, but an incompatible change in Checkmk 1.6.

Note that some devices report "0 minutes left" in case they are on mains. In this case no levels are applied, to avoid false positives.

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