Pull out the common bits of InstanceList into _BaseInstanceList
Pull out the common bits of InstanceList into _BaseInstanceList

This is preparing for InstanceList v2.0 and is kept separate to prove
that tests continue to pass before and after this split.

Related to blueprint liberty-bump-object-and-rpcapi-versions

Change-Id: I22fb15440aca9c5c47d718f36997ddd4b7c0c2ee

gooddata/openstack-nova,scripnichenko/nova,vmturbo/nova,vmturbo/nova,rajalokan/nova,MountainWei/nova,Francis-Liu/animated-broccoli,TwinkleChawla/nova,openstack/nova,klmitch/nova,j-carpentier/nova,edulramirez/nova,openstack/nova,cernops/nova,Juniper/nova,dims/nova,yosshy/nova,felixma/nova,fnordahl/nova,yosshy/nova,ted-gould/nova,devendermishrajio/nova,bigswitch/nova,noironetworks/nova,phenoxim/nova,phenoxim/nova,j-carpentier/nova,hanlind/nova,cyx1231st/nova,bigswitch/nova,jianghuaw/nova,ted-gould/nova,rahulunair/nova,gooddata/openstack-nova,vmturbo/nova,NeCTAR-RC/nova,raildo/nova,rahulunair/nova,zhimin711/nova,MountainWei/nova,rajalokan/nova,sebrandon1/nova,alaski/nova,vmturbo/nova,JianyuWang/nova,zhimin711/nova,klmitch/nova,CEG-FYP-OpenStack/scheduler,mikalstill/nova,mahak/nova,klmitch/nova,alaski/nova,BeyondTheClouds/nova,openstack/nova,hanlind/nova,dims/nova,apporc/nova,jianghuaw/nova,JianyuWang/nova,NeCTAR-RC/nova,Juniper/nova,hanlind/nova,cloudbase/nova,cloudbase/nova,TwinkleChawla/nova,cyx1231st/nova,fnordahl/nova,Juniper/nova,scripnichenko/nova,rajalokan/nova,cloudbase/nova,BeyondTheClouds/nova,Juniper/nova,cernops/nova,sebrandon1/nova,rajalokan/nova,devendermishrajio/nova,sebrandon1/nova,raildo/nova,jianghuaw/nova,noironetworks/nova,felixma/nova,mikalstill/nova,mahak/nova,mahak/nova,rahulunair/nova,cernops/nova,apporc/nova,gooddata/openstack-nova,BeyondTheClouds/nova,edulramirez/nova,CEG-FYP-OpenStack/scheduler,klmitch/nova,mikalstill/nova,CCI-MOC/nova,gooddata/openstack-nova,jianghuaw/nova,Francis-Liu/animated-broccoli,CCI-MOC/nova