Use the login from oauth instead of the (optional) name

When a user is created, the login is currently positioned by default to the name. This is bad because the name field is optional (especially in GitHub), resulting in an incorrect user being created.

This PR fetches the login from oauth, and uses it as the username (=login) in SonarQube. Also, if the name is not set, it is positioned to the login.

Note : not tested with Google, as it requires a public-facing website.