models: allow configuration of RRD HEARTBEAT
models: allow configuration of RRD HEARTBEAT

On some configuration, that is very important to be able to set it. 300s
is not always enough (example: a node posting metrics every 5 minutes.
Sometimes, that will be 301 seconds. RRD detect then an error).

jlaine/django-timegraph,jlaine/django-timegraph