Fix simple access group control in case-insensitive domains
    
    https://fedorahosted.org/sssd/ticket/1713
    
    In the simple access provider, we need to only canonicalize user names when
    comparing with values in the ACL, not when searching the cache. The sysdb
    searches might do a base search with a DN constructed with the username
    which fails if the username is lower case.