BigSwitch: Don't import portbindings_db until use
BigSwitch: Don't import portbindings_db until use

The Big Switch ML2 driver re-uses large parts of
code from the Big Switch plugin. However, the plugin
uses the old portbindings_db in its port tracking
code. This db is not created/used by ML2, so it does
not exist when the Big Switch ML2 driver is being used.
This caused a problem when the portbindings_db module
would be imported even though it wasn't being used
since the SQLAlchemy models would create the
relationships during the import.

This patch moves the portbindings_db import down to
the methods that leverage it so it won't be imported
during use by the ML2 driver.

This is a temporary solution which can be back-ported
if disabling auto schema generation is back-ported
to icehouse. An appropriate fix will require some
refactoring to avoid the port tracking code from
being imported for ML2.

Closes-Bug: #1300628
Change-Id: I551abf9d6ec832c423cba0d50bb27715b43c0b1c

dhanunjaya/neutron,cisco-openstack/neutron,takeshineshiro/neutron,silenci/neutron,openstack/neutron,yuewko/neutron,JianyuWang/neutron,jumpojoy/neutron,dims/neutron,blueboxgroup/neutron,yanheven/neutron,SamYaple/neutron,Metaswitch/calico-neutron,vbannai/neutron,jumpojoy/neutron,klmitch/neutron,virtualopensystems/neutron,javaos74/neutron,sasukeh/neutron,yamahata/tacker,alexandrucoman/vbox-neutron-agent,redhat-openstack/neutron,cernops/neutron,aristanetworks/neutron,vbannai/neutron,openstack/neutron,mandeepdhami/neutron,antonioUnina/neutron,yanheven/neutron,cloudbase/neutron-virtualbox,mattt416/neutron,gopal1cloud/neutron,leeseuljeong/leeseulstack_neutron,rdo-management/neutron,wenhuizhang/neutron,blueboxgroup/neutron,dhanunjaya/neutron,projectcalico/calico-neutron,magic0704/neutron,pnavarro/neutron,leeseulstack/openstack,klmitch/neutron,mahak/neutron,CiscoSystems/neutron,magic0704/neutron,gkotton/neutron,blueboxgroup/neutron,swdream/neutron,sebrandon1/neutron,chitr/neutron,bigswitch/neutron,gkotton/neutron,shahbazn/neutron,jerryz1982/neutron,mattt416/neutron,antonioUnina/neutron,redhat-openstack/neutron,swdream/neutron,cisco-openstack/neutron,adelina-t/neutron,mmnelemane/neutron,vveerava/Openstack,watonyweng/neutron,barnsnake351/neutron,CiscoSystems/neutron,suneeth51/neutron,cloudbase/neutron,skyddv/neutron,jacknjzhou/neutron,neoareslinux/neutron,leeseuljeong/leeseulstack_neutron,jacknjzhou/neutron,jerryz1982/neutron,virtualopensystems/neutron,JioCloud/neutron,igor-toga/local-snat,mmnelemane/neutron,rdo-management/neutron,aristanetworks/neutron,glove747/liberty-neutron,infobloxopen/neutron,shahbazn/neutron,virtualopensystems/neutron,eonpatapon/neutron,neoareslinux/neutron,Metaswitch/calico-neutron,NeCTAR-RC/neutron,noironetworks/neutron,huntxu/neutron,waltBB/neutron_read,huntxu/neutron,cernops/neutron,javaos74/neutron,leeseulstack/openstack,skyddv/neutron,dims/neutron,cloudbase/neutron,vveerava/Openstack,eayunstack/neutron,waltBB/neutron_read,SamYaple/neutron,suneeth51/neutron,paninetworks/neutron,mahak/neutron,projectcalico/calico-neutron,sasukeh/neutron,barnsnake351/neutron,mahak/neutron,NeCTAR-RC/neutron,Stavitsky/neutron,wolverineav/neutron,apporc/neutron,vivekanand1101/neutron,takeshineshiro/neutron,asgard-lab/neutron,gkotton/neutron,yamahata/tacker,Stavitsky/neutron,eayunstack/neutron,leeseuljeong/leeseulstack_neutron,asgard-lab/neutron,bigswitch/neutron,CiscoSystems/neutron,alexandrucoman/vbox-neutron-agent,MaximNevrov/neutron,miyakz1192/neutron,watonyweng/neutron,igor-toga/local-snat,apporc/neutron,wolverineav/neutron,miyakz1192/neutron,glove747/liberty-neutron,vveerava/Openstack,pnavarro/neutron,bgxavier/neutron,JioCloud/neutron,JianyuWang/neutron,cloudbase/neutron-virtualbox,eonpatapon/neutron,yamahata/tacker,SmartInfrastructures/neutron,chitr/neutron,paninetworks/neutron,openstack/neutron,noironetworks/neutron,leeseulstack/openstack,silenci/neutron,vivekanand1101/neutron,infobloxopen/neutron,sebrandon1/neutron,mandeepdhami/neutron,vbannai/neutron,bgxavier/neutron,yuewko/neutron,wenhuizhang/neutron,MaximNevrov/neutron,SmartInfrastructures/neutron,gopal1cloud/neutron,adelina-t/neutron