Use random network in boot_server scenario
Use random network in boot_server scenario

In this scneario we are going to run a lot of VMs one by one.
Usually one network has about 500 fixed ips (or we will get performance issues)

So if we are going to run 2k vms we should use at least 4 networks.

To simplify configuration of this test. We will by default chose
random network to get ip for vm.

Change-Id: If5d8c52b4f02315be2bcc542baf43cdcc0989316

yeming233/rally,ytsarev/rally,cernops/rally,vponomaryov/rally,go-bears/rally,amit0701/rally,shdowofdeath/rally,paboldin/rally,go-bears/rally,openstack/rally,openstack/rally,pyKun/rally,aforalee/RRally,pandeyop/rally,gluke77/rally,aplanas/rally,gluke77/rally,vponomaryov/rally,openstack/rally,amit0701/rally,vganapath/rally,vefimova/rally,vganapath/rally,vganapath/rally,gluke77/rally,pyKun/rally,yeming233/rally,varunarya10/rally,eonpatapon/rally,eayunstack/rally,eonpatapon/rally,group-policy/rally,paboldin/rally,paboldin/rally,cernops/rally,aforalee/RRally,ytsarev/rally,afaheem88/rally,vefimova/rally,gluke77/rally,vganapath/rally,redhat-openstack/rally,aplanas/rally,group-policy/rally,group-policy/rally,openstack/rally,amit0701/rally,eayunstack/rally,pandeyop/rally,shdowofdeath/rally,eayunstack/rally,redhat-openstack/rally,afaheem88/rally,varunarya10/rally