Add a field to enter the subjectAlternativeName in when creating the request certificate.

The RFC6125 (https://tools.ietf.org/html/rfc6125) recommends

Move away from including and checking strings that look like
      domain names in the subject's Common Name.

   o Move toward including and checking DNS domain names via the
      subjectAlternativeName extension designed for that purpose:
      dNSName.
   o Move toward including and checking even more specific
      subjectAlternativeName extensions where appropriate for using the
      protocol (e.g., uniformResourceIdentifier and the otherName form
      SRVName).

   o Move away from the issuance of so-called wildcard certificates
      (e.g., a certificate containing an identifier for
      "*.example.com").

So add that field in the request, and suggest people to use the system hostname in there.