MINIFI-513 Clear secret key when auto restarting in order to obtain new secret key from the MiNiFi process

This closes #172.

Signed-off-by: Aldrin Piri <aldrin@apache.org>
diff --git a/minifi-bootstrap/src/main/java/org/apache/nifi/minifi/bootstrap/RunMiNiFi.java b/minifi-bootstrap/src/main/java/org/apache/nifi/minifi/bootstrap/RunMiNiFi.java
index 2fb4e0b..93d5f05 100644
--- a/minifi-bootstrap/src/main/java/org/apache/nifi/minifi/bootstrap/RunMiNiFi.java
+++ b/minifi-bootstrap/src/main/java/org/apache/nifi/minifi/bootstrap/RunMiNiFi.java
@@ -1388,6 +1388,7 @@
                             setNiFiStarted(false);
                         }
 
+                        secretKey = null;
                         process = builder.start();
                         handleLogging(process);