Restore use of management.health.*.enabled keys

The move of health.* keys to management.health.* broke them as they
are not defined as configuration keys and `ManagementServerProperties`
is strict on the whole management namespace.

This commit updates the tests to actually include that properties class
and relax the "ignoreUnknownFields" condition so that extra attributes
can be defined on the "management" namespace.

Fixes gh-2115