MaxChecksumAge cannot be 0 in settings

Before IntegrityService checks a pillar for ObsoleteChecksums, it checks whether the MaxChecksumAge for the pillar is 0. If that is the case, then the pillar is ignored. (Especially useful to prevent obsolete checksum on ChecksumPillars).

But in the settings, the MaxChecksumAge is defined as PositiveInteger, which means that it is not possible to set it to 0.

Change type from 'xs:positiveInteger' into 'xs:nonNegativeInteger'