Allow forms to work directly, and no longer require explicit overwrite

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |no  |
|New Feature? |yes  |
|BC Breaks?   |yes |
|Deprecations?|no  |
|Tests Pass?  |yes  |
|Fixed Tickets|   |
                   
This change allows a user-system to reuse the default form types.

When a form-type begins with 'fos_user' prefix the UserServicesFactory will register the form-type with the service-prefix (and it own name), using the RollerworksMultiUserBundle form types which allow a customizable name.

**In short:** the user-bundle is no longer required to always define its own form classes in order to work properly. Which will make bootstrapping a new user-bundle a little easier.
And of course you can still use your own form-types as before.

One important BC breaking change is that each form-name is prefixed with the service prefix and not the fos_user prefix (as before), even when your only using one user-system.
This is not a big problem, but may break your functional tests. :boom: