Configs correctly cleaned, even if phpunit fails or autotest interrupted

document docker parameter and use random host port
fix typo
copy autotest.sh to autotest-external.sh
adds ability to add start* and stop* scripts in env in external tests
run files_external WebDAV tests against ownCloud instance
introduce executor number to be able to shut down the correct docker container
fetch docker images in advance - this also fetches latest versions of the docker images
add second argument to autotest-external.sh which can specify a single test to run
print out the explicit test run
change naming schema of files_external setup scripts

When hitting Ctrl+C to interrupt unit test running, the old
configuration was not restored properly.

This fix traps the signal to properly restore the configuration after an
interruption.

error: pathspec 'tests/data/lorem-copy.txt' did not match any file(s) known to git.
error: pathspec 'tests/data/testimage-copy.png' did not match any file(s) known to git.

This will prevent frustration and confusion when unit tests fail because
the wrong phpunit version was used

autotest.sh now checks for the validity of the db config name argument.
If the db config name is not known, it show syntax info.

This should save some time to people to understand how to use this
script. Before this fix, an invalid argument would return a big HTML
page in the output.

The script now checks for config.php existance and backs it up before
running the test, then restores it back at the end. This avoids the
situation where devs lose their manually edited config if they forgot to
back it up before running the unit tests.

The script now also checks for config.php permissions beforehand to
avoid displaying an annoying HTML page output in the console with an
error message.

enabling, because the test shows a green ball: https://ci.tmit.eu/job/ownCloud-Server(doctrine)-oci/33/console