Refactors pantheon private key to signing private key (#34)

Signed-off-by: Joshua Richardson <joshua@richardson.tech>
Signed-off-by: Lucas Saldanha <lucas.saldanha@consensys.net>

acceptance-tests/dsl/src/main/java/org/hyperledger/besu/tests/acceptance/dsl/privacy/PrivacyNode.java

@@ -195,7 +195,7 @@ public class PrivacyNode implements AutoCloseable {
   }
 
   public String getTransactionSigningKey() {
-    return besu.keyPair().getPrivateKey().toString();
+    return besu.getPrivacyParameters().getSigningKeyPair().orElseThrow().getPrivateKey().toString();
   }
 
   public void addOtherEnclaveNode(final URI otherNode) {