Ensures that the Linux Bridge Plugin runs with L3 agent
Ensures that the Linux Bridge Plugin runs with L3 agent

Fixes bug 1048968

1. The external interface for the linux bridge will be created at run time.
The configuration parameter external_network_bridge should be set as empty.
That is, "external_network_bridge = ". The link between the network and
the physical interface should be done via the provider network parameters.
2. The plug operation for the linux bridge replaces the interface name
prefix with tap. We need to ensure that the prefix length is 3 characters.

Change-Id: Ib8e26bc1f603a1aff9560b4a40110a6824db5921

Juniper/contrail-dev-neutron,Brocade-OpenSource/OpenStack-DNRM-Neutron,ykaneko/quantum,skyddv/neutron,cloudbase/neutron-virtualbox,mahak/neutron,rossella/neutron,Stavitsky/neutron,aristanetworks/arista-ovs-quantum,bgxavier/neutron,JianyuWang/neutron,takeshineshiro/neutron,oeeagle/quantum,vbannai/neutron,kaiweifan/vse-lbaas-plugin-poc,aristanetworks/arista-ovs-quantum,miyakz1192/neutron,vijayendrabvs/hap,gkotton/neutron,bigswitch/neutron,aristanetworks/neutron,yamt/neutron,wenhuizhang/neutron,vveerava/Openstack,suneeth51/neutron,ntt-sic/neutron,sajuptpm/neutron-ipam,kaiweifan/neutron,oeeagle/quantum,virtualopensystems/neutron,MaximNevrov/neutron,blueboxgroup/neutron,liqin75/vse-vpnaas-plugin,cloudbase/neutron-virtualbox,sasukeh/neutron,yamahata/neutron,sasukeh/neutron,Comcast/neutron,rickerc/neutron_audit,watonyweng/neutron,yamahata/tacker,psiwczak/quantum,vbannai/neutron,liqin75/vse-vpnaas-plugin,mandeepdhami/neutron,javaos74/neutron,miyakz1192/neutron,netscaler/neutron,armando-migliaccio/neutron,psiwczak/quantum,yamt/neutron,pnavarro/neutron,JioCloud/neutron,bigswitch/neutron,jumpojoy/neutron,apporc/neutron,tpaszkowski/quantum,javaos74/neutron,neoareslinux/neutron,vbannai/neutron,CiscoSystems/vespa,eayunstack/neutron,CiscoSystems/quantum,silenci/neutron,kaiweifan/vse-lbaas-plugin-poc,blueboxgroup/neutron,ntt-sic/neutron,ykaneko/quantum,Juniper/contrail-dev-neutron,armando-migliaccio/neutron,wolverineav/neutron,jumpojoy/neutron,chitr/neutron,cisco-openstack/neutron,barnsnake351/neutron,mahak/neutron,glove747/liberty-neutron,SamYaple/neutron,leeseulstack/openstack,gopal1cloud/neutron,rossella/neutron,Juniper/neutron,infobloxopen/neutron,gopal1cloud/neutron,mandeepdhami/neutron,sebrandon1/neutron,armando-migliaccio/neutron,takeshineshiro/neutron,rdo-management/neutron,armando-migliaccio/neutron,cernops/neutron,NeCTAR-RC/neutron,openstack/neutron,ykaneko/quantum,eonpatapon/neutron,kaiweifan/vse-lbaas-plugin-poc,Juniper/contrail-dev-neutron,dhanunjaya/neutron,zhhf/charging,antonioUnina/neutron,openstack/neutron,adelina-t/neutron,mattt416/neutron,aristanetworks/arista-ovs-quantum,magic0704/neutron,Comcast/neutron,alexandrucoman/vbox-neutron-agent,CiscoSystems/quantum,rdo-management/neutron,klmitch/neutron,yamahata/neutron,CiscoSystems/neutron,vijayendrabvs/ssl-neutron,mmnelemane/neutron,SamYaple/neutron,igor-toga/local-snat,chitr/neutron,rickerc/neutron_audit,Metaswitch/calico-neutron,FreescaleSemiconductor/quantum,rickerc/neutron_audit,JianyuWang/neutron,rossella/neutron,gkotton/neutron,swdream/neutron,yuewko/neutron,vveerava/Openstack,aristanetworks/arista-ovs-quantum,cloudbase/neutron,asgard-lab/neutron,netscaler/neutron,cisco-openstack/neutron,vveerava/Openstack,tpaszkowski/quantum,leeseulstack/openstack,eayunstack/neutron,yamahata/neutron,Brocade-OpenSource/OpenStack-DNRM-Neutron,sajuptpm/neutron-ipam,paninetworks/neutron,NeCTAR-RC/neutron,dims/neutron,CiscoSystems/vespa,kaiweifan/neutron,antonioUnina/neutron,kaiweifan/vse-lbaas-plugin-poc,beagles/neutron_hacking,noironetworks/neutron,ykaneko/quantum,blueboxgroup/neutron,tpaszkowski/quantum,CiscoSystems/neutron,vijayendrabvs/ssl-neutron,vivekanand1101/neutron,FreescaleSemiconductor/quantum,swdream/neutron,citrix-openstack-build/neutron,vijayendrabvs/ssl-neutron,beagles/neutron_hacking,watonyweng/neutron,dims/neutron,netscaler/neutron,yamt/neutron,waltBB/neutron_read,infobloxopen/neutron,sajuptpm/neutron-ipam,wenhuizhang/neutron,wolverineav/neutron,Brocade-OpenSource/OpenStack-DNRM-Neutron,apporc/neutron,citrix-openstack-build/neutron,jacknjzhou/neutron,dhanunjaya/neutron,projectcalico/calico-neutron,citrix-openstack-build/neutron,eonpatapon/neutron,mahak/neutron,tpaszkowski/quantum,virtualopensystems/neutron,neoareslinux/neutron,gkotton/neutron,silenci/neutron,liqin75/vse-vpnaas-plugin,shahbazn/neutron,aristanetworks/neutron,JioCloud/neutron,leeseuljeong/leeseulstack_neutron,ykaneko/neutron,FreescaleSemiconductor/quantum,redhat-openstack/neutron,jerryz1982/neutron,Stavitsky/neutron,CiscoSystems/neutron,FreescaleSemiconductor/quantum,ykaneko/neutron,suneeth51/neutron,huntxu/neutron,SmartInfrastructures/neutron,leeseulstack/openstack,klmitch/neutron,vivekanand1101/neutron,kaiweifan/neutron,mmnelemane/neutron,CiscoSystems/quantum,shahbazn/neutron,yanheven/neutron,zhhf/charging,Juniper/neutron,yamahata/tacker,cloudbase/neutron,noironetworks/neutron,redhat-openstack/neutron,skyddv/neutron,MaximNevrov/neutron,bgxavier/neutron,projectcalico/calico-neutron,barnsnake351/neutron,magic0704/neutron,leeseuljeong/leeseulstack_neutron,virtualopensystems/neutron,waltBB/neutron_read,ntt-sic/neutron,Juniper/neutron,ykaneko/neutron,yanheven/neutron,liqin75/vse-vpnaas-plugin,sebrandon1/neutron,cernops/neutron,psiwczak/quantum,Metaswitch/calico-neutron,leeseuljeong/leeseulstack_neutron,asgard-lab/neutron,alexandrucoman/vbox-neutron-agent,yamt/neutron,pnavarro/neutron,vijayendrabvs/hap,yamahata/tacker,zhhf/charging,SmartInfrastructures/neutron,jacknjzhou/neutron,huntxu/neutron,igor-toga/local-snat,mattt416/neutron,CiscoSystems/vespa,Comcast/neutron,beagles/neutron_hacking,psiwczak/quantum,rossella/neutron,jerryz1982/neutron,vijayendrabvs/hap,openstack/neutron,glove747/liberty-neutron,adelina-t/neutron,paninetworks/neutron,yuewko/neutron