enable unit tests for provisioning api

fix admin docu key

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

Tried to add more apps (others break).
"preprocessors" is now populated automatically based on the tested apps.

Paths without the leading slash apply to all subdirectories, which especially
means that /tests/data is ignored, making it unnecessarily hard to add test
data.