Provivde reqCert to ScepServlet implementation

If reqCert has been issued previously by the CA, we can use it to update the certificate.

From draft:
If the requesting system already has a certificate issued by the CA, that certificate SHOULD be presented as credentials for the renewal of that certificate if the CA supports the "Renewal" capability and the CA policy permits the certificate to be renewed.

Cisco Docs:
As the expiration date of an ID certificate approaches, a SCEP client might want to obtain a new certificate. The client generates a CSR and goes through the Enrollment process (as defined previously). The current certificate is used in order to sign the SignedData PKCS#7, which in turn proves identity to the CA. Upon reciept of the new certificate, the client immediately deletes the current certificate and replaces it with the new one, whose validity starts immediately.