Prevent thrashing when deploying many bm instances
Prevent thrashing when deploying many bm instances

Nova BM is easily overwhelmed at the moment because it has to convert
every disk image to raw - simply deploying 2 instances at once exceeds
the IOPS on most single disk servers. Avoid this by serialising the
image conversion (which leads to a lower average-time-to-deploy in
nearly all situations).

Change-Id: I6d5ee97ffdbd497802379b13e70b84d70d8ded57
Co-Authored-By: Steve Kowalik <4068f0880b399410602d694b3cc711c8a8f4727e@wedontsleep.org>

maelnor/nova,nikesh-mahalka/nova,CEG-FYP-OpenStack/scheduler,varunarya10/nova_test_latest,petrutlucian94/nova,cloudbase/nova-virtualbox,edulramirez/nova,saleemjaveds/https-github.com-openstack-nova,gooddata/openstack-nova,tealover/nova,raildo/nova,mandeepdhami/nova,cyx1231st/nova,CCI-MOC/nova,JioCloud/nova_test_latest,mahak/nova,mahak/nova,affo/nova,Yusuke1987/openstack_template,j-carpentier/nova,tianweizhang/nova,mahak/nova,CiscoSystems/nova,kimjaejoong/nova,dawnpower/nova,ted-gould/nova,zzicewind/nova,BeyondTheClouds/nova,viggates/nova,Tehsmash/nova,iuliat/nova,rajalokan/nova,BeyondTheClouds/nova,affo/nova,noironetworks/nova,alexandrucoman/vbox-nova-driver,blueboxgroup/nova,cernops/nova,cloudbase/nova-virtualbox,badock/nova,bgxavier/nova,badock/nova,devendermishrajio/nova,virtualopensystems/nova,CEG-FYP-OpenStack/scheduler,redhat-openstack/nova,jianghuaw/nova,vmturbo/nova,tianweizhang/nova,NeCTAR-RC/nova,cernops/nova,klmitch/nova,vmturbo/nova,JianyuWang/nova,maelnor/nova,LoHChina/nova,adelina-t/nova,silenceli/nova,eonpatapon/nova,felixma/nova,Juniper/nova,jeffrey4l/nova,leilihh/novaha,scripnichenko/nova,mmnelemane/nova,jeffrey4l/nova,gooddata/openstack-nova,JioCloud/nova_test_latest,alaski/nova,alaski/nova,leilihh/novaha,isyippee/nova,MountainWei/nova,blueboxgroup/nova,barnsnake351/nova,ruslanloman/nova,klmitch/nova,tangfeixiong/nova,shail2810/nova,redhat-openstack/nova,fnordahl/nova,bigswitch/nova,alvarolopez/nova,Metaswitch/calico-nova,eharney/nova,thomasem/nova,rahulunair/nova,hanlind/nova,mgagne/nova,Yusuke1987/openstack_template,zhimin711/nova,belmiromoreira/nova,klmitch/nova,zaina/nova,iuliat/nova,BeyondTheClouds/nova,TwinkleChawla/nova,zaina/nova,jianghuaw/nova,akash1808/nova,rrader/nova-docker-plugin,eayunstack/nova,spring-week-topos/nova-week,barnsnake351/nova,akash1808/nova_test_latest,watonyweng/nova,TwinkleChawla/nova,raildo/nova,sebrandon1/nova,bgxavier/nova,rahulunair/nova,alexandrucoman/vbox-nova-driver,varunarya10/nova_test_latest,watonyweng/nova,spring-week-topos/nova-week,shahar-stratoscale/nova,shail2810/nova,rahulunair/nova,Metaswitch/calico-nova,Francis-Liu/animated-broccoli,tanglei528/nova,apporc/nova,virtualopensystems/nova,whitepages/nova,vladikr/nova_drafts,berrange/nova,CiscoSystems/nova,yatinkumbhare/openstack-nova,mikalstill/nova,gooddata/openstack-nova,mandeepdhami/nova,luogangyi/bcec-nova,vladikr/nova_drafts,phenoxim/nova,JianyuWang/nova,cernops/nova,alvarolopez/nova,dims/nova,klmitch/nova,takeshineshiro/nova,Francis-Liu/animated-broccoli,JioCloud/nova,sebrandon1/nova,viggates/nova,fnordahl/nova,ewindisch/nova,tangfeixiong/nova,gooddata/openstack-nova,bigswitch/nova,leilihh/nova,openstack/nova,tealover/nova,joker946/nova,eayunstack/nova,cloudbase/nova,devendermishrajio/nova_test_latest,eharney/nova,thomasem/nova,scripnichenko/nova,berrange/nova,Stavitsky/nova,vmturbo/nova,JioCloud/nova,ewindisch/nova,felixma/nova,tudorvio/nova,belmiromoreira/nova,projectcalico/calico-nova,cloudbase/nova,Juniper/nova,devendermishrajio/nova,zhimin711/nova,openstack/nova,NeCTAR-RC/nova,tanglei528/nova,yatinkumbhare/openstack-nova,mikalstill/nova,ted-gould/nova,tudorvio/nova,jianghuaw/nova,isyippee/nova,Tehsmash/nova,j-carpentier/nova,rajalokan/nova,cloudbase/nova,kimjaejoong/nova,rajalokan/nova,ruslanloman/nova,orbitfp7/nova,Juniper/nova,akash1808/nova,rrader/nova-docker-plugin,angdraug/nova,leilihh/nova,whitepages/nova,openstack/nova,silenceli/nova,nikesh-mahalka/nova,rajalokan/nova,LoHChina/nova,petrutlucian94/nova,saleemjaveds/https-github.com-openstack-nova,sebrandon1/nova,hanlind/nova,vmturbo/nova,CloudServer/nova,apporc/nova,cyx1231st/nova,luogangyi/bcec-nova,MountainWei/nova,yosshy/nova,angdraug/nova,phenoxim/nova,CloudServer/nova,noironetworks/nova,projectcalico/calico-nova,dawnpower/nova,double12gzh/nova,shahar-stratoscale/nova,zzicewind/nova,devendermishrajio/nova_test_latest,joker946/nova,mikalstill/nova,CCI-MOC/nova,yosshy/nova,adelina-t/nova,double12gzh/nova,Juniper/nova,edulramirez/nova,eonpatapon/nova,hanlind/nova,akash1808/nova_test_latest,mmnelemane/nova,dims/nova,jianghuaw/nova,Stavitsky/nova,takeshineshiro/nova,mgagne/nova,orbitfp7/nova