Cleanup of jenkins init scripts run on EC2 VMs

- Add SSH known_hosts for gerrit instances
- Don't run apt-get update as jenkins doesn't run as root or have
  sudo rights to add/update packages.
- Cleanup and commenting of what each section does

Change-Id: I89571bb988a211b226c0b8568b3c1f8ad7226340
diff --git a/jenkins-scripts/jenkins-init-script.sh b/jenkins-scripts/jenkins-init-script.sh
index 67987b5..8adf422 100755
--- a/jenkins-scripts/jenkins-init-script.sh
+++ b/jenkins-scripts/jenkins-init-script.sh
@@ -13,10 +13,15 @@
 
 cd /ci-management/jenkins-scripts
 chmod +x ./*.sh
-./system_type.sh
 
+# create system type script and source it
+./system_type.sh
 source /tmp/system_type.sh
+
+# run basic settings
 ./basic_settings.sh
+
+# run per system-type script, if it exists
 if [ -f "${SYSTEM_TYPE}.sh" ]
 then
     ./"${SYSTEM_TYPE}.sh"