Fix tls-auth/crypt in connection blocks with --persist-key

Fix tls-auth/crypt in connection blocks with --persist-key

If --persist-key was used, we would always try to pre-load the 'global'
tls-auth/crypt file. That would result in using the wrong key (leading
to a failed connection) or en error is there was to 'global' key:

  Sat Jan 19 11:09:01 2019 Cannot pre-load tls-auth keyfile ((null))
  Sat Jan 19 11:09:01 2019 Exiting due to fatal error

Fix that by loading loading the key from the current connection entry.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20190119103400.12887-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18123.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>