add an image_list tool
add an image_list tool

this is an interface from devstack to provide a more robust
listing of the possible image urls that we might ask for during
a devstack run. This allows tools like nodepool, or 3rd party CI
systems to precache all this content in advance.

Change-Id: I7474f553ecf28e51a2340bef5bcefa962dbadc24

promptworks/devstack,HPCHub/devstack,upadhyay-prashant/devstack,noironetworks/devstack,vsham20/devstack,NaokiMiyata/devstack,samgoon/devstack,vishnugonela/devstack,nawawi/openstack,pacinete/devstack,liuquansheng47/devstack,thomasem/devstack,dtroyer/devstack,bigswitch/devstack,HPCHub/devstack,TwinkleChawla/devstack,eshnil2000/devstack,sequenceiq/devstack,olivierlemasle/devstack,Juniper/devstack,liuquansheng47/devstack,olivierlemasle/devstack,herrsechs/devstack,nuxeh/devstack,affo/devstack,eharney/devstack,bq-xiao/devstack,atulpatil301/devstack,sequenceiq/devstack,ozamiatin/devstack,NaokiMiyata/devstack,LoHChina/devstack,Mahito/devstack,richliu/devstack,affo/devstack,bq-xiao/devstack,dirkmueller/devstack,avvocatodemarchis/devstack,mssumanth/devstack,wenhuizhang/devstack,sc68cal/devstack,bljgaurav/openstack-test,vsham20/devstack,promptworks/devstack,kplus/devstack-moonshot,sc68cal/devstack,varunarya10/devstack,andrewrothstein/devstack,atulpatil301/devstack,rushiagr/devstack,CloudServer/devstack,group-policy/devstack,nuxeh/devstack,innodee/openstackk,TwinkleChawla/devstack,upadhyay-prashant/devstack,parul24/devstack,richliu/devstack,openstack-dev/devstack,parul24/devstack,pacinete/devstack,thomasem/devstack,srics/devstack,jamielennox/devstack,ozamiatin/devstack,openstack-dev/devstack,dtroyer/devstack,pombredanne/devstack,wenhuizhang/devstack,andrewrothstein/devstack,Mahito/devstack,LoHChina/devstack,avvocatodemarchis/devstack,tianweizhang/devstack,bigswitch/devstack,neerja28/Devstack_GlusterFS,pczerkas/devstack,BSJAIN92/OpenStack,kplus/devstack-moonshot,bljgaurav/openstack-test,noironetworks/devstack,mmasaki/devstack,flavio-fernandes/devstack,costingalan/devstack,prithivm/branch01,j4/devstack,virtualopensystems/devstack,eharney/devstack,flavio-fernandes/devstack,vishnugonela/devstack,neerja28/Devstack_GlusterFS,pczerkas/devstack,mc2014/devstack,BSJAIN92/OpenStack,costingalan/devstack,tianweizhang/devstack,samgoon/devstack,prithivm/branch01,CloudServer/devstack,pombredanne/devstack,williamthegrey/devstack,nawawi/openstack,j4/devstack,dirkmueller/devstack,varunarya10/devstack,herrsechs/devstack,group-policy/devstack,mc2014/devstack,bswartz/devstack,mssumanth/devstack,bswartz/devstack,mmasaki/devstack,rushiagr/devstack