Migrate to use v0.14 API

[cosmo0920]

This PR patches should be treated as major update.
Fluentd core development team strongly recommended to use v0.14 API, and I've tried to use v0.14 API in this plugin.

BTW, I'm using Debian stretch (9.0) in development, and I've found that @with_systemd mechanism does not work for my Debian environment.

My environment, does not have "#{@cgroup_path}/#{metric_type}/system.slice/docker-#{id}.scope/#{metric_filename}" path.
Instead, it has "#{@cgroup_path}/#{metric_type}/docker/#{id}/#{metric_filename}" path.
This issue is caused by my misconfiguration?

% ls /lib/systemd/system/docker.service
/lib/systemd/system/docker.service
% ls /etc/init.d/docker
/etc/init.d/docker
% cat /etc/debian_version
9.0

Thanks,

[cosmo0920]

ping?
Fluentd v0.14 and its successor v1 had been stablized for almost 1 year:
https://www.fluentd.org/blog/fluentd-v1.0

[cosmo0920]

Any progress?