fixed role creation/relation when adding a new user

when creating a new user, a role is associated with all locales; 
bugfix for: https://github.com/sulu-cmf/sulu-standard/issues/70
if roles already exists, the first one found is taken (lowest id), else a new one called "Default" is created

Tasks:
- [x] gather feedback for my changes


| Q             | A
| ------------- | ---
| Bug fix?      | yes 
| New feature?  | no
| Tests pass?   | none
| Fixed tickets | yes
| Doc           | none