Revert "Increase parallelism to 8 builds"

Apparently we are hitting the limits of the Gerrit master
to keep up in provisioning the slaves.

Need to reduce the speed and investigate further in the meantime.

This reverts commit 606512bb40d923b819660f9fb57da5ed66661a3f.

Change-Id: I482b6737d49f3bcc8b5562d7102b14cc988006c3
diff --git a/jenkins/gerrit-verifier-flow.groovy b/jenkins/gerrit-verifier-flow.groovy
index 63811f3..4779d63 100644
--- a/jenkins/gerrit-verifier-flow.groovy
+++ b/jenkins/gerrit-verifier-flow.groovy
@@ -33,7 +33,7 @@
   static int numRetryBuilds = 3
   static int myAccountId = 1022687
   static int waitForResultTimeout = 10000
-  static int maxBuilds = 8
+  static int maxBuilds = 4
   static String verifierJobName = "Gerrit-verifier-change"
 }