Fix multiple additional flags for docker_network

[lemrouch]

I tried to create docker network with multiple additional parameters like:
docker_network { 'db_net'
..
additional_flags => '--scope swarm --attachable --internal --config-from db_net_config',
..
but Docker complained about uknown flag:

Error: Execution of '/usr/bin/docker network create --driver=macvlan --scope swarm --attachable --internal --config-from db_net_config db_net' returned 125: unknown flag: --scope swarm --attachable --internal --config-from db_net_config
See 'docker network create --help'.

While running the command by simple copy&paste worked just fine:

~# /usr/bin/docker network create --driver=macvlan --scope swarm --attachable --internal --config-from db_net_config db_net
ep37zquecbh4qp5ks06n1v244

Each additional flag has to be pushed as extra String otherwise
Docker will complain about uknown flag.

I don't know how to write tests but even clean repository fails bundle exec rake spec on Ubuntu 19.04 with:

Failures:

1. create creates a docker network
   Failure/Error: expect(provider.create).to be_nil

 NoMethodError:
   undefined method `create' for nil:NilClass
 # ./spec/unit/lib/puppet/provider/docker_network_spec.rb:22:in `block (3 levels) in <top (required)>'

[sheenaajay]

Thanks @lemrouch for submitting the PR. Could you please add the new flag name here.
https://github.com/puppetlabs/puppetlabs-docker/blob/9575cfde1cac752240e3c9b946c19f6c5c7302c5/spec/unit/lib/puppet/type/docker_network_spec.rb
Please let us know if you need more information. Thank you.