win: allow OpenVPN service account to use any command-line options

win: allow OpenVPN service account to use any command-line options

Since 2.7, OpenVPN service (used to start persistent connections)
runs under limited virtual service account NT SERVICE\OpenVPNService.

Since it should be able to use all command-line options
and cannot be made member of "OpenVPN Administrators" group,
it has to be handled separately.

Change-Id: I44d308301dfb7c22600d8632a553288f52b3068f
Signed-off-by: Lev Stipakov <lev@openvpn.net>