Changed the way the user redirection urls are generated

Changed the way the user redirection urls are generated in the `contentCreateViewService` allowing users to overwrite the default values of the redirection attributes using plugins or another services.
As written in https://github.com/ezsystems/PlatformUIBundle/pull/144#discussion_r22345359, it's `value`, not `valueFn` as we want to set here a function that will be evaluated later (in the getter `_redirectionUrl`).
`valueFn` is a function to set a default value of an attribute, that's a bit different and you don't control when the `valueFn` function will be evaluated.