Update to foreman_bootdisk 3.0.0

Please only release for nightly (1.6).
Yeah, I've been trying to keep the RPMs and plugin requirements in sync.  I'm tempted to lock them down fairly conservatively, e.g. saying that a plugin is only valid for ~> 1.6.0, as many plugins we know break between major versions.  Using a conservative dependency means we can force the package to get upgraded when they upgrade Foreman by releasing an updated plugin.