volume/driver: make unit test, test_volume, pass
volume/driver: make unit test, test_volume, pass

fake command executer doesn't return command result.
Which return None instead of string.
So add None check to make unit test pass.

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