authentication script example with one-time password for OpenVPN

need to add to OpenVPN config:

auth-user-pass-verify /PATH_TO_SCRIPT/openvpn_oath_verify.pl via-file
script-security 2
username-as-common-name # without this option openvpn will use cn in the certificate as username

it wotks with eTokens, applications like "Google Authenticator", etc.