Setup a docker container that holds a webdav instance to test files_external

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