yumostree: Add altfiles to nsswitch.conf automatically
yumostree: Pile of changes

* Remove modes other than 'create' for now.  While it would be really
  nice to support mutating the current system, that's a lot harder.

* Switch to 2 phase bootstrap; we need to install nss-altfiles etc.
  and create /usr/lib/passwd, so that later %post write to it

* Run dracut here, after creating /etc/machine-id