BE: Change be_refresh_ctx_init to return errno and set be_ctx->refresh_ctx
    
    It is a bit odd that a caller to a be_ function would set a property of
    be_ctx. IMO it is cleaner if the function has a side-effect and sets the
    property internally and rather returns errno.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/4012
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>