preload: minikube continues before preload download is over

[medyagh]

this is from head on docker driver:

to replicate : first purge

medmac@~/workspace/minikube (master) $ ./out/minikube delete --all --purge
🔥  Successfully deleted all profiles
💀  Successfully purged minikube directory located at - [/Users/medmac/.minikube]

then start with docker driver

medmac@~/workspace/minikube (master) $ ./out/minikube start --vm-driver=docker 
😄  minikube v1.8.1 on Darwin 10.13.6
✨  Using the docker driver based on user configuration
💾  Downloading preloaded images tarball for k8s v1.17.3 ...
🔥  Creating Kubernetes in docker container with (CPUs=2) (2 available), Memory=4000MB (7964MB available) ...
    > preloaded-images-k8s-v1-v1.17.3-docker-overlay2.tar.lz4: 499.26 MiB / 499
🐳  Preparing Kubernetes v1.17.3 on Docker 19.03.2 ...
    ▪ kubeadm.pod-network-cidr=10.244.0.0/16
❌  Unable to load cached images: loading cached images: stat /Users/medmac/.minikube/cache/images/gcr.io/k8s-minikube/storage-provisioner_v1.8.1: no such file or directory
    > kubelet.sha256: 65 B / 65 B [--------------------------] 100.00% ? p/s 0s
    > kubeadm.sha256: 65 B / 65 B [--------------------------] 100.00% ? p/s 0s
    > kubectl.sha256: 65 B / 65 B [--------------------------] 100.00% ? p/s 0s
    > kubeadm: 37.52 MiB / 37.52 MiB [----------------] 100.00% 8.85 MiB p/s 4s
    > kubectl: 41.48 MiB / 41.48 MiB [----------------] 100.00% 8.15 MiB p/s 6s
    > kubelet: 106.42 MiB / 106.42 MiB [-------------] 100.00% 14.14 MiB p/s 8s
🚀  Launching Kubernetes ... 

I believe this happnes to only docker driver because docker driver creates much faster than others. while for VMs while the VMs are booting up the preload finish.

That makes me think may be for VM worlds we could have a warm up VM (while the big download is happnenining we could warm up the VM) (for another issue)

[medyagh]

CC: @priyawadhwa