Improve REST API tests

We want to improve our test coverage on Nuxeo Platform when using REST API.

Currently a lot of our features are tested against JSF and use webdriver tests.
In order to prepare JSF removal, and so webdriver removal, we need to add more functional tests on REST API.

We need:

• to create or use tools to follow Nuxeo Platform coverage (only REST coverage)
• to rework our functional test framework to face new usage.
  current one uses RestHelper which is a wrapper of nuxeo-java-client in order to not expose it directly in Nuxeo Platform
• to define what features are gonna be tested and the REST workflow we use from WEB UI