Setup a docker container that holds a webdav instance to test files_exte...

...rnal

cc @DeepDiver1975 @PVince81 

Let's wait for an exploding Jenkins :)

Used docker container: https://registry.hub.docker.com/u/morrisjobke/webdav/ Source: https://github.com/MorrisJobke/owncloud-docker/tree/master/webdav
another idea would be to make the shell script generate a config_webdav.php script fully, then make PHP use that script by default. This instead of having a single file with all configs, have one config file per storage type.