TT#44919 Change "openssl genrsa" so it works with buster and older releases

The interface of the "openssl genrsa" changed in Debian buster (10), and it
doesn't accept "-config" or "-batch" any longer.  It is not very clearly
documented (or easy to find) how and when, because versions of 1.1.0 still work
in the old way, but in 1.1.1 it doesn't.

The new sub-command "genpkey" is supposed to supersede genrsa, but the output
file is slightly different, genrsa having RSA in the BEGIN/END blocks, while
genpkey not having info about the algorithm:

  -----BEGIN RSA PRIVATE KEY-----
  vs.
  -----BEGIN PRIVATE KEY-----

This small change could bite us if we're parsing these files in some way, and I
found comments complaining that some services do not like the "new" header if
they are linked against older versions of openssl, so e.g. they (Exim in
particular) fail to start.

I found a way in which it works fine for both versions, by still using "genrsa"
and passing the config file through an environment parameter, and removing
-batch.

After pondering about it for a bit, I prefer this solution to another one
parsing the Debian release or openssl version and using one or other version
accordingly, specially because of the lack of clarity of exactly when the
versions had changed.

The rest of the commands continue to accept -config and -batch, so in principle
we do not need to change them, for the time being, but probably we will have to
revisit them in the long run, specially if they keep breaking the interface
without much of a fallback or clear warning.

Change-Id: I495d8cccf120c5a2665d9843392fe580ec59f4bd
(cherry picked from commit 8f9f92eb68)
(cherry picked from commit 25302d185f)

tools/generate_ssl_keys.sh

@@ -47,8 +47,8 @@ if [ "$SKIP_CSR" = "true" ] ; then
        -out "${CRT_FILE}"          \
        -nodes -batch
 else
-  /usr/bin/openssl genrsa -out "${KEY_FILE}" 4096 \
-    -config "${OPENSSL_CONFIG}" -batch
+  OPENSSL_CONF="${OPENSSL_CONFIG}" /usr/bin/openssl genrsa \
+    -out "${KEY_FILE}" 4096
   /usr/bin/openssl req -new -out "${CSR_FILE}" \
     -key "${KEY_FILE}" -config "${OPENSSL_CONFIG}" -batch
   /usr/bin/openssl x509 -days "${DAYS}" -req -in "${CSR_FILE}" \