Use -addext when generating CSR with OpenSSL 1.1.1

[spaze]

OpenSSL 1.1.1 has added a new option to req to specify SAN (https://github.com/openssl/openssl/issues/3311):

-addext "subjectAltName = DNS:foo.co.uk" \

Let's use that instead of creating a new temp file when OpenSSL 1.1.1 is used.