deb-upgrade.sh does not fetch Packages file from releases

in deb-upgrade.sh script there is logic which retrieves a list of packages from the Packages file:

package_list=$(grep -B 1 'Source: mariadb-' Packages | grep 'Package:' | grep -vE 'galera|spider|columnstore' | awk '{print $2}' | sort | uniq | xargs)

It wasn't used before, as it is only done for minor upgrades in "all packages" mode, while the buildbot only runs the minimal upgrade. We need to enable the "all" mode now, in the scope of MDBF-800.

For upgrade tests the list of packages should be coming from the last release (unlike in installation tests, where it comes from the build which is being tested).

However, there is no logic in upgrade tests which fetches the Packages file. It used to exist, but at some point when the scripts were re-written, it got lost.

In the old buildbot, the file is wget-ed from a mirror (as it was in the scripts before the change). It can also be done differently, e.g. by setting up the repo and doing apt-get update, and then finding the file in the cache.

How the list is retrieved, isn't important, as long as it works.

Here is how it looks when it doesn't work:

grep: Packages: No such file or directory

++ grep '^Package:' Packages

++ grep -vE 'galera|spider|columnstore'

++ xargs

++ awk '{print $2}'

grep: Packages: No such file or directory

+ package_list=

Instead, package_list should be initialized to a long list of package names.