Skip port mappings when no changes are detected

[erikvdv1]

The dokku_ports task always completes with changed=true, even when the same ports are already set for an application.

This PR checks if the existing ports match the ports to set, if so, it completes with changed=false.

The command for checking existing ports dokku --quiet proxy:ports {app} returns a non-zero exit code when no ports are defined, so I replaced this with a call to dokku --quiet proxy:report {app}.

dokku --quiet proxy:report {app}
       Proxy enabled:                 true
       Proxy port map:                http:80:80 https:443:80
       Proxy type:                    nginx