Fix FWaaS race condition

The current FWaaS manifest suffers from a race condition: it may
attempt to configure the FWaaS service before the Neutron L3 agent
package on which it depends has been installed, and before the
/etc/neutron directory exists.  This leads to errors on the first
catalog run, and the FWaaS service doesn't start.  The errors
go away on the second catalog run since by then the requisite
directory exists.  This patch adds ordering such that the L3
agent package gets installed before the FWaaS service is configured.