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.

This is needed for Dune modules that use git submodules
to initialized the submodules and potentially even their
submodules.

`dunecontrol bexec` does not execute it's argument directly, but passes it
through `eval`, effectively removing one layer of quotes

Specify test labels to build and run tests for in the environment variable
DUNECI_TEST_LABELS.  Multiple labels can be seperated by '|', ',', or
whitespace.

This makes it easier to update the CI environment.

The base-10 image is currently using clang 4, which has major compatibility issues with the stdlib
of GCC 7 in C++17 mode, mostly revolving around std::string_view. This makes it impossible to use
the clang in this image for compiling Dune.

This patch resolves the issue by simply bumping clang to version 5.

Ubuntu 16.10 is EOL.  Rebuilding the image fails as the repository is
no longer present.

The Debian packages do not have the most recent changes.

PDELab needs dune-alugrid for a lot of its test. As we don't want to have dune-alugrid in the
default Dune images (those should only contain stuff from core and staging), build separate base
images for PDELab.

This add libscotchparmetis, libgmp and libeigen to the list of pre-installed external libraries.

Reference: https://gitlab.dune-project.org/infrastructure/issues0/issues/48

But only if `${DUNECI_PARALLEL}` is set.

Reference: https://gitlab.dune-project.org/infrastructure/issues0/issues/47

Reference: https://gitlab.dune-project.org/infrastructure/issues0/issues/44

For testing dune-corepy.

This is useful for the weekly automatic rebuild of the images.  These
should always rebuild the images so updated packages from Debian get
included.