Fixes bug OPENSTACK-640
Fixes bug OPENSTACK-640

Added a check on startup of the plugin to verify that the default_fip_rate
paramter is a valid value.

Change-Id: Ic0ca0e89122bec53b19df0247fec016fc1fb7a2d
Closes-Bug: OPENSTACK-640

nuagenetworks/nuage-openstack-neutron,naveensan1/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron,naveensan1/nuage-openstack-neutron