Rename SSL_CERT_FILE to NODED_CERT_FILE

To be consistent with RAPI_CERT_FILE, the rather generic named
“SSL_CERT_FILE” constant is renamed to “NODED_CERT_FILE”. The actual file
name is not changed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>