[fluentd-custom] Support `test-kitchen` and also Ubuntu 14.04.

- Add `.kitchen.yml`.
- Add `Berksfile`.
- Do not configure IPTABLES settings, when testing.
- add tests

As `td-agent` does not surppot Ubuntu 14.04,
install `td-agent` version 2, instead of version 1.

Due to this modification, use `td_agent_gem` resource.
This `td_agent_gem` resource automatically detects which `gem` binary for `td-agent` to use,
and install the required fluentd plugins, using that `gem` binary.