2.0: consider rename of oci_tarball

[alexeagle]

Now that it doesn't produce a tarball as a default output, the name is confusing.

Also since it's a noun, it seems like it should be build'able - but the recommended use is to run it for the side-effect of loading the layers into a daemon. So it should have a verb name like oci_load or oci_daemon_load.

While we do that we should probably rename the output group to oci_tarball so the migration path from 1.x is more clear.

[thesayyn]

fixed by #636

[alexeagle]

Note, we could also pre-declare an output like [name].tar so you're not forced to go through the filegroup trick to request it. OTOH we want to discourage actually running tar to produce a huge output from an action, so I guess inconvenience is a feature.