Add WOPI integration tests

We want to validate continuously and automatically that the WOPI Office integration is working in the latest version of Nuxeo Server.

Let's:

• Set up a dedicated Jenkins X builder, for instance python-wopi, with pip/setuptools and the WopiValidatorExecutor, see https://github.com/microsoft/wopi-validator-cli-python.
• Write the wopi/Jenkinsfile:
  • Start Nuxeo with the nuxeo-wopi package installed.
  • Upload a .wopitest file using the REST API (curl).
  • Generate an access token with JWT and a shared secret put in the nuxeo configuration.
  • Call the Python validator with the WopiSrc of the .wopitest file and a valid WOPI AccessToken.
  • Archive the wopivalidatorexecutor.log file.
  • We might need to parse it to "whitelist" some tests expected to fail since related to not implemented features.
  • Send a Slack notification in #platform-notifs.

Note: let's make sure the Python validator is covering the same stuff as the Web one.