fuel_upgrade: add ability to use diff-based repo

In Fuel 6.0 we're going to introduce diff-based releases, so the
fuel-upgrade script has to handle and install it properly.

(cherry picked from commit 21a21ed2acfa42aebf0f6551fa3329c9f2b00522)

Blueprint: reduce-tarball-size
Change-Id: If05ac74b680691bf9adb54caf8958e235ec767ef
Merge "fuel_upgrade: add ability to use diff-based repo" into stable/5.1