gitlab-runner now writes traces to the file system and thus needs to
be able to write somewhere.

The DUNE CI images are built on gitlab.dune-project.org now.

Otherwise Ubuntu 18.04 otherwise prompts for input when installing tzdata.

This patch introduces a new shell variable DUNECI_TEST_INSTALL. If this variable is set in
`.gitlab-ci.yml`, the script will try to run `make install` before building and running the unit
tests.

This just uses /duneci/install as installation prefix for now.

This should make the label parsing function easier for the human reader.

Ansgar prefers a pattern of the form
```sh
VAR=default_value
if explicitly_configured; then
  VAR=configured_value
fi
```
This restructures the labels parsing the make this pattern possible.