Add Helper for automatic definition of skeleton folder

[patrickjahns]

Currently the skeleton directory is set from external via run.sh - https://github.com/owncloud/core/blob/master/tests/acceptance/run.sh#L596-L625

It makes sense to have a specific helper (api) that accept 3 values:

• api
• ui
• "path"

So we can configure it from the outside if needed, but when api/ui is defined, it assumes that we use the folder provided from the location where the testing app resides