* provider/ignition: migration from resources to data resources
* website: provider/ignition documention updated to data resources
* provider/ignition: backwards compatibility support for old resources
According to the coreos [documentation](https://coreos.com/ignition/docs/latest/configuration.html), systemd units only require the name attribute per each unit. This can also be validated with the CoreOS config validator. This change allows the `ignition_systemd_unit` resource to no longer fail if given an empty `content` and `dropin`.
Also adds a test to cover this use case.