Add docker CoreOS support

Add support CoreOS to vagrant-proxyconf docker function.
This is indicated in #74.

This modification is based on #75.
Only c31a4ba is added to #75.

It has been tested and the result is OK.
I think this test should be done as root, as `systemctl` might not be in the vagrant user's PATH. I.e. something like this:

```ruby
@export = (@machine.communicate.test('which systemctl', sudo: true) ? '' :  'export ')
```