testiso: Add support for testing live path too

The `testiso` command now supports testing both `live-initramfs`
and `initramfs` (legacy installer).  The default is to test
a live system.  To test legacy, use e.g.:

`kola testiso --cosa-build builds/latest/x86_64/meta.json -SKL`

Also, pass through other arguments via the old `cosa test-coreos-installer`
wrapper.

TODO:
- Support testing the "just run Live" case - maybe try to figure out
  how to have main `kola` tests apply?
- Test `coreos-install iso embed` path

Author: cgwalters