add Ganglia cluster and grid heartbeat alerts

$ riemann-client query 'service="grid heartbeat"'
Event #0:
  time  = 1392396072 - Fri Feb 14 16:41:12 2014
  state = (null)
  service = grid heartbeat
  host = PROD:FlexibleContent
  description = seconds
  ttl = 80.000000
  metric_sint64 = 1392396068
  metric_d = 0.000000
  metric_f = 1392396032.000000
  tags = [ cloud=GWS/GC2 os=Linux stage=PROD role=flxadm datacentre=dc1a ]
  attributes = {
    grid = FlexibleContent
    cluster = n/a
    environment = PROD
    resource = FlexibleContent
    location = dc1a
    index-time = 1392395696
    ip = 10.252.39.82
  }