Fix ironic-python-agent image not loading vfat mod

Closes-Bug: 1589450
Load the vfat driver as a Pre Exec action for systemd before starting
ironic-python-agent in order to allow reading of parameters.txt file
required for the ironic-python-agent to find its configuration.

Change-Id: Ibf74dd1b2678ea76e0676711a7aa5ba6b88d5421

elements/ironic-agent/install.d/ironic-agent-source-install/ironic-python-agent.service

@@ -3,6 +3,7 @@ Description=Ironic Python Agent
 After=network-online.target
 
 [Service]
+ExecStartPre=/usr/sbin/modprobe vfat
 ExecStart=/usr/local/bin/ironic-python-agent
 Restart=always
 RestartSec=30s