[infra] teach create_bootstrap_vm.sh to use random names (#11310)
[infra] teach create_bootstrap_vm.sh to use random names (#11310)

In the unlikely change that the user already has a VM called boostrap-vm,
this will significantly reduce the likelihood of a name-clash.
hail-is/hail,hail-is/hail,hail-is/hail,hail-is/hail,hail-is/hail,hail-is/hail,hail-is/hail,hail-is/hail