Add openssl to 1.16 docker image

commit 72694652:

docker/cerbero: Install openssl-devel manually
Temporarily. This will be pulled in by cerbero bootstrap once
https://gitlab.freedesktop.org/gstreamer/cerbero/merge_requests/367 is
merged.

commit 50912478:

docker/cerbero: Do not fail while removing artifacts
The artifacts may not exist if, f.ex., the build job failed in the
previous pipeline.

commit 4c480203:

docker/cerbero: Clone the correct cerbero branch when building
We need to pull in the bootstrap deps of the 1.16 branch, not the
master branch.

I verified that an image built with these changes will work correctly and pass the cerbero fedora job here: https://gitlab.freedesktop.org/nirbheek/gst-ci/-/jobs/1282509

Needed for cerbero!367 (merged).