Use individual access keys for yubikeys

[cornelinux]

as an enhancement for #23 we could add individual access keys for each yubikey.

privacyideaadm could generate random access keys.
privacyIDEA could store these indivual token access keys in the SOPIN in the token table.
If the yubikey ever needed to be reprogrammed, then privacyidea would know the access key of the yubikey.

If the token would be deleted from the database, the access key would however be lost.