Add warning if both vm-driver and driver are specified

[priyawadhwa]

New output:

$ minikube start
😄  minikube v1.9.0-beta.1 on Darwin 10.14.6
❗  Both driver=hyperkit and vm-driver=virtualbox have been set.
			
    Since vm-driver is deprecated, minikube will default to driver=hyperkit.

    If vm-driver is set in the global config, please run "minikube config unset vm-driver" to resolve this warning.	
			
✨  Using the hyperkit driver based on user configuration
🔥  Creating hyperkit VM (CPUs=2, Memory=6000MB, Disk=20000MB) ...

Should fix #7091

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: priyawadhwa

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [priyawadhwa]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov-io]

Codecov Report

Merging #7109 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #7109      +/-   ##
==========================================
- Coverage   37.19%   37.17%   -0.03%     
==========================================
  Files         146      146              
  Lines        9106     9112       +6     
==========================================
  Hits         3387     3387              
- Misses       5303     5309       +6     
  Partials      416      416

Impacted Files	Coverage Δ
cmd/minikube/cmd/start.go	37.31% <0%> (-0.39%)	⬇️