Triple verify changes parallelism to leverage extra slaves

There are now 6 slaves with 4 parallel VMs executors each,
which gives us triple power for running many more parallel
changes verifications at the same time.

Change-Id: Ic9180b1bcf1d55e62245f24813c67f27db300407
diff --git a/jenkins/gerrit-verifier-flow.groovy b/jenkins/gerrit-verifier-flow.groovy
index 4779d63..302d8c6 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 = 4
+  static int maxBuilds = 12
   static String verifierJobName = "Gerrit-verifier-change"
 }