ts(1): digest option is mandatory

not specifying the digest both on command line and in the config file
will lead to response generation aborting with

140617514493760:error:2F098088:time stamp routines:ts_CONF_lookup_fail: \
    cannot find config variable:crypto/ts/ts_conf.c:106:tsr_test::signer_digest

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8421)

(cherry picked from commit 29716a03e8c602d69d6c86558b2903b23edddd9b)