fuel_upgrade: add hack to make repo symlink

We need to add next symlink

    repo/ubuntu/x86_64 -> repo/ubuntu/fuelweb/x86_64

in order to get working upgrade to 6.0, because in 6.0 we know nothing
about 'fuelweb' folder in repo struct, but we have it in old repos from
previous installation.

Change-Id: I7985a1f64b54bfff84df7098252448ea6ec6b613
Closes-Bug: #1390521
Merge "fuel_upgrade: add hack to make repo symlink"