project_maven: allow to set private repository

The values map to these GUI choices

 * default            -> Default
 * local-to-executor  -> Local to the executor
 * local-to-workspace -> Local to the workspace

This is on the project level what the similar option is on the builder
level.

Change-Id: I90ee3385ee12a46b5ab1eb26e3af1bdbfc36946a
Merge "project_maven: allow to set private repository"