Removing value in the --prune parameter for docker stack deploy

[venushka]

Removing value in the --prune parameter for docker stack deploy as the cli doesn't have a value as per the documentation. (see https://docs.docker.com/engine/reference/commandline/stack_deploy/)

With the existing code, if we use the prune option as follows, the docker stack deploy command fails.

  docker::stack { 'proxy': 
    ensure        => present,
    stack_name    => 'proxy',
    compose_files => '/tmp/proxy-compose.yml',
    prune         => true
  }

[davejrt]

LGTM

[venushka]

Thanks for merging this in.