Adds vxlan kernel module to Heat template

Previously, the vxlan kernel module was not being loaded. This
causes the following error when trying to start the lb-agent
when using the vxlan type driver:

ERROR neutron.plugins.linuxbridge.agent.linuxbridge_neutron_agent
[-] Linux kernel vxlan module and iproute2 3.8 or above are
required to enable VXLAN.

Change-Id: I1c9f5d13d25e887d15270a96d35f41a08316c8f0

devenv/kollanode.yaml

@@ -182,6 +182,9 @@ resources:
             tar -xvf version-m3.tar.gz
             mv kolla-version-m3 kolla
 
+            # Add vxlan kernel module for Neutron
+            modprobe vxlan
+
             # Start NTP
             systemctl enable ntpd
             systemctl start ntpd