BE: Make be_refresh_ctx_init set up the periodical task, too
    
    This is mostly a preparatory patch that rolls in setting up the ptask
    into be_refresh_ctx_init. Since in later patches we will call
    be_refresh_ctx_init from several different places, this will prevent
    code duplication.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/4012
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>