Make  ComputeTestCase.test_state_revert faster
Make  ComputeTestCase.test_state_revert faster

Reuse instance instead of creating a new one every time.

Before:
nova.tests.compute.test_compute.ComputeTestCase.test_state_revert 11.032 seconds
After:
nova.tests.compute.test_compute.ComputeTestCase.test_state_revert 6.415 seconds

Change-Id: I67a5a971c3898d4fffed9e6b4640c6c0f3c62a56

apporc/nova,cloudbau/nova,OpenAcademy-OpenStack/nova-scheduler,imsplitbit/nova,leilihh/nova,ntt-sic/nova,sacharya/nova,rahulunair/nova,akash1808/nova_test_latest,badock/nova,affo/nova,shail2810/nova,joker946/nova,sridevikoushik31/openstack,thomasem/nova,openstack/nova,mahak/nova,NeCTAR-RC/nova,ntt-sic/nova,rahulunair/nova,alaski/nova,Triv90/Nova,mgagne/nova,tudorvio/nova,shootstar/novatest,mikalstill/nova,vladikr/nova_drafts,sridevikoushik31/openstack,eayunstack/nova,fnordahl/nova,alaski/nova,JianyuWang/nova,openstack/nova,virtualopensystems/nova,NeCTAR-RC/nova,orbitfp7/nova,klmitch/nova,dawnpower/nova,sebrandon1/nova,mgagne/nova,alexandrucoman/vbox-nova-driver,yosshy/nova,ted-gould/nova,berrange/nova,watonyweng/nova,eonpatapon/nova,mmnelemane/nova,eharney/nova,TwinkleChawla/nova,SUSE-Cloud/nova,vmturbo/nova,j-carpentier/nova,qwefi/nova,yrobla/nova,Brocade-OpenSource/OpenStack-DNRM-Nova,devendermishrajio/nova,redhat-openstack/nova,luogangyi/bcec-nova,edulramirez/nova,qwefi/nova,DirectXMan12/nova-hacking,bgxavier/nova,rajalokan/nova,akash1808/nova,cyx1231st/nova,rajalokan/nova,luogangyi/bcec-nova,rajalokan/nova,tealover/nova,adelina-t/nova,Yusuke1987/openstack_template,affo/nova,isyippee/nova,adelina-t/nova,TwinkleChawla/nova,Stavitsky/nova,fnordahl/nova,CEG-FYP-OpenStack/scheduler,gooddata/openstack-nova,maheshp/novatest,jeffrey4l/nova,blueboxgroup/nova,tanglei528/nova,j-carpentier/nova,Tehsmash/nova,viggates/nova,badock/nova,varunarya10/nova_test_latest,phenoxim/nova,angdraug/nova,Metaswitch/calico-nova,projectcalico/calico-nova,imsplitbit/nova,rickerc/nova_audit,maheshp/novatest,hanlind/nova,JioCloud/nova_test_latest,BeyondTheClouds/nova,TieWei/nova,citrix-openstack-build/nova,belmiromoreira/nova,openstack/nova,scripnichenko/nova,mahak/nova,alvarolopez/nova,eharney/nova,JianyuWang/nova,rrader/nova-docker-plugin,zhimin711/nova,klmitch/nova,tealover/nova,apporc/nova,joker946/nova,berrange/nova,dstroppa/openstack-smartos-nova-grizzly,petrutlucian94/nova,noironetworks/nova,nikesh-mahalka/nova,cloudbase/nova-virtualbox,LoHChina/nova,zzicewind/nova,MountainWei/nova,blueboxgroup/nova,cloudbase/nova,Juniper/nova,mmnelemane/nova,cyx1231st/nova,JioCloud/nova,bgxavier/nova,yatinkumbhare/openstack-nova,double12gzh/nova,tianweizhang/nova,OpenAcademy-OpenStack/nova-scheduler,cernops/nova,barnsnake351/nova,dstroppa/openstack-smartos-nova-grizzly,Tehsmash/nova,maelnor/nova,klmitch/nova,dims/nova,tanglei528/nova,phenoxim/nova,takeshineshiro/nova,CloudServer/nova,DirectXMan12/nova-hacking,barnsnake351/nova,belmiromoreira/nova,Juniper/nova,tianweizhang/nova,devendermishrajio/nova_test_latest,mahak/nova,rajalokan/nova,spring-week-topos/nova-week,takeshineshiro/nova,ewindisch/nova,tangfeixiong/nova,iuliat/nova,kimjaejoong/nova,yatinkumbhare/openstack-nova,alexandrucoman/vbox-nova-driver,SUSE-Cloud/nova,felixma/nova,maheshp/novatest,projectcalico/calico-nova,felixma/nova,Stavitsky/nova,Yuriy-Leonov/nova,leilihh/novaha,leilihh/nova,eayunstack/nova,alvarolopez/nova,vladikr/nova_drafts,jeffrey4l/nova,jianghuaw/nova,vmturbo/nova,vmturbo/nova,zaina/nova,raildo/nova,gspilio/nova,plumgrid/plumgrid-nova,sebrandon1/nova,Juniper/nova,DirectXMan12/nova-hacking,bclau/nova,gspilio/nova,cernops/nova,Metaswitch/calico-nova,devendermishrajio/nova_test_latest,gspilio/nova,ewindisch/nova,CiscoSystems/nova,sridevikoushik31/nova,sridevikoushik31/nova,citrix-openstack-build/nova,cloudbase/nova,sacharya/nova,ted-gould/nova,rahulunair/nova,plumgrid/plumgrid-nova,Francis-Liu/animated-broccoli,cloudbase/nova,CEG-FYP-OpenStack/scheduler,ruslanloman/nova,gooddata/openstack-nova,devendermishrajio/nova,TieWei/nova,sridevikoushik31/nova,shahar-stratoscale/nova,zhimin711/nova,virtualopensystems/nova,viggates/nova,saleemjaveds/https-github.com-openstack-nova,CloudServer/nova,spring-week-topos/nova-week,whitepages/nova,jianghuaw/nova,petrutlucian94/nova_dev,klmitch/nova,JioCloud/nova,rrader/nova-docker-plugin,gooddata/openstack-nova,shail2810/nova,mikalstill/nova,Yuriy-Leonov/nova,shahar-stratoscale/nova,bclau/nova,ruslanloman/nova,dstroppa/openstack-smartos-nova-grizzly,zzicewind/nova,zaina/nova,mandeepdhami/nova,sebrandon1/nova,vmturbo/nova,yosshy/nova,noironetworks/nova,akash1808/nova_test_latest,kimjaejoong/nova,bigswitch/nova,leilihh/novaha,devoid/nova,BeyondTheClouds/nova,double12gzh/nova,Brocade-OpenSource/OpenStack-DNRM-Nova,cernops/nova,MountainWei/nova,CiscoSystems/nova,mikalstill/nova,orbitfp7/nova,maelnor/nova,jianghuaw/nova,akash1808/nova,dawnpower/nova,JioCloud/nova_test_latest,Triv90/Nova,redhat-openstack/nova,hanlind/nova,CCI-MOC/nova,iuliat/nova,thomasem/nova,saleemjaveds/https-github.com-openstack-nova,isyippee/nova,gooddata/openstack-nova,tangfeixiong/nova,Juniper/nova,eonpatapon/nova,CCI-MOC/nova,yrobla/nova,LoHChina/nova,edulramirez/nova,mandeepdhami/nova,cloudbau/nova,sridevikoushik31/nova,BeyondTheClouds/nova,varunarya10/nova_test_latest,rickerc/nova_audit,Yusuke1987/openstack_template,tudorvio/nova,hanlind/nova,jianghuaw/nova,bigswitch/nova,petrutlucian94/nova,whitepages/nova,angdraug/nova,Triv90/Nova,dims/nova,petrutlucian94/nova_dev,silenceli/nova,silenceli/nova,scripnichenko/nova,sridevikoushik31/openstack,Francis-Liu/animated-broccoli,watonyweng/nova,devoid/nova,nikesh-mahalka/nova,raildo/nova,yrobla/nova,cloudbase/nova-virtualbox,shootstar/novatest