Multiple unused heat-engine are left after the deployment

With multiple Heat units, a lot of unused (down) heat-engine will be shown up in `openstack orchestration service list`. It would be nice if Heat charm cleans up after a deployment or as an charm action like cinder charm action of `remove-services`.

$ openstack orchestration service list | grep -w up -c
42

openstack orchestration service list | grep -w down -c
42

$ openstack orchestration service list | grep juju-8ce0c5-0-lxd-8
| juju-8ce0c5-0-lxd-8 | heat-engine | 4b1c2bb7-b7f7-4ea7-86da-668989b06f2e | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | b1ef9cb6-4fc1-401e-a39e-c2f9275c8477 | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | 3ef8f77f-1106-4815-8392-e1757e4027f2 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | f2464220-ee5a-424d-b216-f9dd6f5ec450 | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | e43b7633-5d5c-48d4-bec2-fb1bf49d8dbf | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | c3041f40-01d0-42ea-8f48-9e973c261cc1 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | a30edcad-fda9-4d64-ad57-707a2ea4bdf6 | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | a354d66b-f563-4b65-91fd-13dcda152979 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | d8ef67e0-e2f6-40bc-b426-46b3c3adf59e | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | d640d2d0-0e39-46c0-85ce-cc14c802b6f2 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 2d9b121c-f46d-44c9-84e5-3938a1104a99 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 5d836128-cc7f-42e1-b04a-4a6d4a0ef6fe | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | d506aff5-864f-4e33-b53f-241ed7124f92 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 3dcc8381-ad2a-43fd-ba44-db23c03516b0 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 5a7d94ca-186f-4b63-a713-b96b9405c587 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 11051e40-14b0-44ba-a198-1c4cc99238bd | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | 2f3712b4-3a74-44ab-b1f2-947dfef6914c | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | 31f4584b-2af7-4705-86d0-2a4dc1938046 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | c86567af-af31-4ea0-b63e-17bfa0de8e5e | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | b6732b72-504e-4b99-b027-ad60e40870c1 | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | 8ff3805b-4965-404f-bbe5-d9dee1da6365 | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | 2df84cb4-e49c-4c96-b7de-2ea1fb53c616 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | a54cad28-a82d-4dfb-96e4-1f002ad2d5a1 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 553e5757-a23f-4e8e-b1f6-fd9e8fd5aee9 | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | 3c8a8b09-ee3f-4adb-8a04-c9d3a514a9b3 | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 32553bae-0370-4347-b70a-0db478a1dfac | heat | engine | 2018-03-14T04:05:13.000000 | up |
| juju-8ce0c5-0-lxd-8 | heat-engine | 1f1c52ff-eab9-430b-8a85-39ad27ba96d1 | heat | engine | 2018-03-13T04:43:21.000000 | down |
| juju-8ce0c5-0-lxd-8 | heat-engine | 64b011ef-abc1-4e49-85e0-c38687fa98ef | heat | engine | 2018-03-13T04:43:21.000000 | down |