This fixes A/B upgrades and rollback API.
Installer manifest supports now an option to preserve partition contents
while disk is being re-partitioned and partitions are re-formatted.
Mount `/boot` partition as needed (to find current label before starting
the installation and in the rollback API).
Fix upgrade API for non-master nodes.
Contents of `/boot`, `/system/state` and META partitions are preserved
in memory while the disk is re-partitioned.
Remove `--save` flag from the installer as it's not being used.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
