Default vmdriver selection on existing node shouldn't override existing driver

[balopat]

I would expect minikube to just simply start an existing minikube cluster by:

minikube start -p <clustername> 

However this is not the case when the default vm driver is different from the vmdriver of the node, I saw this error message when restarting my existing docker2 node.

minikube start -p docker2                                                                              1194ms  Thu Feb 27 10:07:32 2020
😄  [docker2] minikube v1.7.3 on Darwin 10.15.3
✨  Automatically selected the hyperkit driver
💥  The existing "docker2" VM that was created using the "docker" driver, and is incompatible with the "hyperkit" driver.
👉  To proceed, either:

    1) Delete the existing "docker2" cluster using: 'minikube -p profile delete'

    * or *

    2) Start the existing "docker2" cluster using: 'minikube -p profile start --vm-driver=docker'
	
💣  Exiting.

[tstromberg]

This is definitely not the intent, and likely a v1.7 regression.