Caching and loading of images is unreliable

[mlgibbons]

Environment: Windows 10 & Windows 7
Minikube version: 0.26.1
OS :Windows
VM Driver: virtual-box
ISO version: 0.26.0

When "minikube start" is executed is tries to download and cache the images under "~/.minikube/cache" which will be required for "kubeadm init". It then transfers them into the VM and does a "docker load" for each image.

Unfortunately the current code has two problems: 1) race condition: the caching of the images is done in an async go routine without a wait to check that caching has completed before the loading phase starts. The loading phase can start before the images are fully cached resulting in an attempt to load incomplete or missing image files; 2) the loading phase also runs as a go routine and errors are not caught. This hides any problems in image loading.

The net result is unpredictable behaviour in "minikube start" which is particularly problematic when trying to perform this operation in offline mode (#2825) where the cache has previously been primed.

[mlgibbons]

Addressed by #2847 which is now merged