Use existing driver by default, improve driver conflict message

[tstromberg]

This PR does a few things:

• Update driver default to match the current profile
• Move driver conflict detection to occur much earlier
• Provide the user actionable feedback for how to workaround the conflict.
• Use the correct driver name in the conflict message
• Cleanup start.go by performing viper.GetString("vm-driver") only once.

Before:

./out/minikube start --vm-driver=hyperkit
😄  minikube v1.4.0-beta.2 on Darwin 10.14.6
😄  Downloading driver docker-machine-driver-hyperkit:
docker-machine-driver-hyperkit: 10.79 MiB / 10.79 MiB  100.00% 40.82 MiB p/s 1s
😄  A new hyperkit driver was installed. It needs elevated permissions to run. The following commands will be executed
...
💣  The existing "minikube" VM was created using the hyperkit driver.

After:

😄  minikube v1.4.0-beta.2 on Darwin 10.14.6
💥  You have an existing "minikube" VM that was created using the "virtualbox" driver, and is incompatible with the "hyperkit" driver.
👉  To proceed, either:
      1) Delete the existing VM using: 'minikube delete'
      or
      2) Restart with the existing driver: 'minikube start --vm-driver=virtualbox'
💣  Exiting due to driver incompatibility

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