common: Fix domain case sensitivity init
    
    The domain case sensitivity was wrongly set in the domain
    context after initialization if the provider was AD.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3235
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 867bb85ecc8117aa8bdde9add0df8857cf87236e)