Add --services flag to compose ps

[aiordache]

Add compose ps flag.

services:
  web:
    image: nginx
    ports:
      - 8080:80
  test1:
    image: nginx
    scale: 3
  test2:
    image: nginx

$ docker compose ps
NAME                  SERVICE             STATUS              PORTS
compose-cli_test1_1   test1               running             80/tcp
compose-cli_test1_2   test1               running             80/tcp
compose-cli_test1_3   test1               running             80/tcp
compose-cli_test2_1   test2               running             80/tcp
compose-cli_web_1     web                 running             0.0.0.0:8080->80/tcp

$ docker compose ps --services
test1
test2
web

Closes #1273

[ndeloof]

LGTM
please avoid extra code duplication.
I hardly understand golang don't provide those in standard library :'(

[ndeloof]

allready declared in ecs/context.go
as well as local/util.go
Those are just a few line, but many better to avoid more duplication
let's move those all into utils package, ok?