When `forceCN` is set in provisioner configuration and
Subject.CommonName is empty, set Subject.CommonName to the first SAN
from the CSR to follow the letsencrypt's boulder behavior. This is done
in order to support system which require certificate's Subject field to
be non-empty.
N.B. certbot does not send Subject in its certificate request and relies
on similar behavior of letsencrypt.
Closes https://github.com/smallstep/certificates/issues/259
Make sure we do not pass domains with asterisk (wildcard) in the middle,
like _acme-challenge.*.example.com to lookupTxt function, but preprocess
domain and remove leading wildcard so we lookup for
_acme-challenge.example.com.
Perform domain normalization for wildcard domains, so we do query
TXT records for _acme-challenge.example.domain instead of
_acme-challenge.*.example.domain when performing DNS-01 challenge. In
this way the behavior is consistent with letsencrypt and records queried
are in sync with the ones that are shown in certbot manual mode.
