Remove "Ignoring --vm-driver" warning

[tstromberg]

Related to #4855 and #4614

This warning is noisy and is always doing the wrong thing. It's raised in two scenarios:

1. User has an existing non-virtualbox VM and runs minikube start - sees warning.

The user has not specified a --vm-driver here. We should quietly do what the user wants and resume their existing VM.

2. User has specified an existing VM, and runs minikube start --vm-driver=xxx

The user has explicitly specified a VM driver. We should error and exit, rather than ignoring their request.

Implementation wise, the most obvious way to do this is change the default driver to auto. If they have an existing config, the automatic default is to use that driver. If not, we can stick with virtualbox and build toward "best available".

[RA489]

/assign @RA489

[tstromberg]

@RA489 - Will this issue be resolved in time for v1.4.0 (Sep 16?)

[RA489]

@tstromberg yeah, sure