fix regression on issue 922

This PR addresses the same problem as the following issue:
https://github.com/sonata-project/SonataAdminBundle/pull/922

As SonataUser uses a parameter for a group name, it's not possible to then configure that group in the sonata_admin->dashboard->groups section of config.yml by using neither sonata_user nor the %sonata.user.admin.groupname% parameters. Trying to do using the parameter version might lead to a crash if SonataAdmin is defined before SonataUser in AppKernel.

This PR is an adaptation of the above mentioned PR to resolve group names as parameters, in order to resolve the above mentioned issue