Changes

treewide: use almalinux instead of centos8

CentOS Linux 8 is EOL, attempting to install packages in it results in
an error. CentOS Linux is a rebuild of RHEL (stable versions). The
CentOS projects recommends to use CentOS Stream instead, which is a
build of the "public development branch for RHEL".

After the early EOL was announced on 2020-12-08, alternative projects
AlmaLinux and Rocky Linux have been established as binary compatible
forks of RHEL 8 (stable versions).

Both Alma and Rocky seem to be solid projects, see related Wikipedia
articles and their sources. Pick Alma and adjust the whole tree to use
the almalinux:8 docker image instead of centos:centos8.

Fix for:
  Error: Failed to download metadata for repo 'appstream': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

Related: SYS#5818
Related: https://www.centos.org/cl-vs-cs/
Related: https://www.centos.org/centos-linux-eol/
Related: https://bugs.centos.org/view.php?id=18394
Related: https://en.wikipedia.org/wiki/AlmaLinux
Related: https://en.wikipedia.org/wiki/Rocky_Linux
Change-Id: I30e1a773b901b1d2187214445116c7f2aecc4e36

ttcn3-pgw-test: make osmo-uecups-daemon more verbose

Change-Id: Id491b4f9db63c82f3dc58f3fc523b4f84fea01e0
Related: SYS#5602

osmo-uecups-master: install iputils-ping for ttcn3-pgw-test

In ttcn3-pgw-test we order osmo-uecups-daemon to execute `ping`,
which is not available in Debian images.  Install it.

Change-Id: I73a78949d2e433d7cc4233395e45665056f083c8
Related: SYS#5602

ttcn3-pgw-test: set PGW_Tests.mp_run_prog_log_path to '/data'

Change-Id: Idd5a37e5768c400d410958149b97869db1e92fb1
Depends: I04ab92c13fcd893d39dbb6a8a8af5ff41d207e36
Related: SYS#5602