iutil: Transparently redirect anyone who asks root=/mnt/sysimage to sysroot

This is a followup from the previous commit to split the physical
mount and OS root path.  No code should be asking to run code directly
in the physical root, so just redirect them to the OS root.
install: Move Payload postInstall() after bootloader

None of the current Payload subclasses are sensitive to ordering with
respect to the bootloader.  The forthcoming OSTreePayload class will
require postprocessing of the bootloader, so let's just swap the
ordering.
Mark kickstart errors as translatable, and hopefully make them more useful too.
Use format strings in the new kickstart error message translations.

Thanks, jenkins.
install: Handle distinct physical root/sysroot

Recent changes introduced the ability to have distinct physical root
and sysroot.  In this case, because we run the bootloader binaries
inside the target, we need to ensure that e.g. /boot is mounted inside
that target root.

Note in this case, we also use a new Blivet API to update its
knowledge of the system root.

This patch also updates the storage write logic to note if
ksdata.ostreesetup.osname is set, because the storage will have to be
written after the packaging.
bootloader: Allow extlinux loader configuration to handle RPMOSTreePayload case

For RPMOSTreePayload, we are just laying down the bootloader
configuration with no kernel - OSTree then updates that configuration.
payload: Add a preBootloader() step

This is necessary for the RPMOSTreePayload subclass - we need to set
up some things after storage has been retargeted, but before the
bootloader is installed.
install: Move /boot to physical root after bootloader install

This is used by the RPMOSTreePayload class - we put /boot in the
target root in order to install the bootloader, now let's undo that
before running any Payload subclasses.
Add RPMOSTreePayload

This is a new backend that uses ostree to install.  However, it's
named RPMOSTree because a future iteration will use rpm-ostree which
will support a hybrid of tree + RPM packages.