Add generate_ephemeral_key that allows a random ephermal key

Add generate_ephemeral_key that allows a random ephermal key

This is useful for features that can use enither a persistent
or an ephemeral key.

Patch V2: Move the functionality of generating a random key into a
          separate function that acts as wrapper for pem_read_key_file
Patch V4: Move wrapper functionality to caller and leave only generate
          epehermal key functionality in the new function
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20190613134834.5709-2-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18527.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>