Exit Firewall Agent if config is invalid

When fwaas config file is not provided to the agent,
but the service is enabled in neutron.conf file the
agent should exit with an error message and should
not proceed further. This patch adds the necessary fix.

Change-Id: Iaced777e3a34e9405050252b17a203689e1c1fc0
Closes-Bug: #1310857
Merge "Exit Firewall Agent if config is invalid"