Nextcloud storage driver (take 2)

[michielbdejong]

Adapted #1863 to latest master

[phil-davis]

https://drone.cernbox.cern.ch/cs3org/reva/2699/1/8

pkg/storage/fs/nextcloud/nextcloud.go:36:2: no required module provides package github.com/cs3org/reva/tests/helpers; to add it:
	go get github.com/cs3org/reva/tests/helpers
make: *** [Makefile:106: build-revad-docker] Error 1

The merge CI failed.

[michielbdejong]

Yes, there was a problem with the Docker build which had gone undetected, see #1999. Sorry for that!
I added a check and fixed it in #2000.