WIP: merge work to interface WOPI with CS3 APIs

[glpatcern]

This is a large PR that brings support for the CS3 APIs. Credits to @LovisaLugnegard for most of the work with to the CS3 APIs python bindings.

Tested so far with the provided unittest file against a Reva daemon running on local storage. The test suite passes entirely.

Important changes on the WOPI Server itself:

• The /wopi/cbox/open call now accepts an opaque storageid + fileid, as well as endpoint + filepath like before.
• User identities are now made generic: xrootd storages enforce a userid to be in uid:gid format, whereas cs3 storages expect a user opaqueid.
• Low-level utilities have been refactored out to a wopiutils.py file.
• Automatic build of a docker image with drone, credits to @SamuAlfageme.

[SamuAlfageme]

@glpatcern we can actually disable the release-unstable pipeline as it was meant to publish the images of a potentially long-lived cs3 branch to test it on k8s. Both master and tags will keep having builds pushed to dockerhub.

[glpatcern]

@glpatcern we can actually disable the release-unstable pipeline as it was meant to publish the

Indeed, makes sense as this branch is going to be merged quite soon now. Let's touch base tomorrow.

[glpatcern]

OK, as it stands I think we've cleared all that could be addressed for now, the only important remaining issue being sciencemesh/sciencemesh#118, which will be addressed by a separate PR.

Also, this branch is live within the CERN network at https://cbox-wopidev-01:8880 - so far so good.

If nothing is outstanding, I'll merge this PR by the end of the day.