For TLS-based EAP types, such as TLS, TTLS, and PEAP, this
optional parameter specifies the name of the file containing the server's
private key. It can be located in the same file as the server certificate
(EAPTLS_CertificateFile). If the private key is
encrypted,which is usually the case,
EAPTLS_PrivateKeyPassword is the key to decrypt
it.
