gitlab-ci: Do not use the standard container images from gitlab

gitlab-ci: Do not use the standard container images from gitlab

Currently all pipelines of the gitlab CI are failing, except for the
"build-user" pipeline. There is an issue with the default container
image (likely Debian stable) where they imported something bad in one
of the system headers:

 /usr/include/linux/swab.h: In function '__swab':
 /builds/huth/qemu/include/qemu/bitops.h:20:34: error: "sizeof" is not
  defined, evaluates to 0 [-Werror=undef]
 #define BITS_PER_LONG           (sizeof (unsigned long) * BITS_PER_BYTE)

We could maybe work-around this issue or wait for the default containers
to get fixed, but considering that we use Ubuntu (and thus Debian-style)
CI in Travis already to a very large extent, we should consider to use
some RPM-based distros in our gitlab CI instead. Thus let's change the
failing pipelines to use Fedora and CentOS (and also one Ubuntu 19.10,
since 20.04 is broken, too) now.

Message-Id: <20200525131823[email protected]>
Acked-by: Alex Bennée <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>