Add support for systemd EnvironmentFile directive

[MiguelV-dev]

I know that an "environment" attribute already exists but it does not fit my needs.

I would like to use the systemd EnvironmentFile directive, in order to modify the environment variables dynamically. After modifying the file I would reload the service configuration and restart the service.

For example, for systemd, here is what I would like to do:

poise_service 'myapp' do
  provider :systemd
  command 'mycommand'
  environment_file '/path/to/my/env.file'
end

And then the systemd template would generate the following:

...
[Service]
EnvironmentFile=/path/to/my/env.file
...

Thanks.

[phubner]

I was looking for this capability recently as well. Would be nice to add to the options variable for systemd.