Fix error at gerrit startup

Rename key configuration api_key into apiKey
Fix configuration issues and update documentation

Fix configuration key:
 * replace api_key by apiKey ("api_key" key is an invalid syntax)
 * use "host" key in place of "url"

Fix commentLink exemple in documentation

Add a documentation section on "set-status" action

Add a documentation section on legacy configuration

Change-Id: I163bd69a017aa73565c92321787084ff1b3ae7a2