[WIP] Configurable admin services

Depends on https://github.com/sonata-project/SonataAdminBundle/pull/1674

Make admin services configurable, similar to what already exists in SonataUserBundle.
Added possibility to hide an admin entry by getting the "show_in_dashboard" from a configurable parameter.
Defaults keep the existing values, so no BC is expected.

WIP: non-orm storage not changed, proper testing, phpunit tests and docs missing.