ref(Health):  auto enable on first REST request
ref(Health):  auto enable on first REST request

...if org.jitsi.jicofo.health.ENABLE_HEALTH_CHECKS
is not set. This also preserves the previous behavior
from before the most recent health checks refactor.

Also fixes an issue where the health checks would return
500 error instead of always success if not enabled what
was written in the docs.

jitsi/jicofo,jitsi/jicofo,jitsi/jicofo,jitsi/jicofo