Use heartbeat attributes for heartbeat alert details

[satterly]

% alerta heartbeat -h
Usage: alerta heartbeat [OPTIONS]

  Send or delete a heartbeat.

  Note: The "environment", "severity", "service" and "group" values are only
  used when heartbeat alerts are generated from slow or stale heartbeats.

Options:
  -O, --origin ORIGIN            Origin of heartbeat.
  -E, --environment ENVIRONMENT  Environment eg. Production, Development
  -s, --severity SEVERITY        Severity eg. critical, major, minor, warning
  -S, --service SERVICE          List of affected services eg. app name, Web,
                                 Network, Storage, Database, Security
  -g, --group GROUP              Group event by type eg. OS, Performance
  -T, --tag TAG                  List of tags eg. London, os:linux, AWS/EC2
  --timeout SECONDS              Seconds before heartbeat is stale
  --customer STRING              Customer
  -D, --delete ID                Delete hearbeat using ID
  -h, --help                     Show this message and exit.

See alerta/alerta#1110 and alerta/alerta#1128