Skip driver autodetection if driver is already set

[tstromberg]

On my MacBook Pro, this PR:

• Speeds up --dry-run by ~9.5X (fixes TestFunctional/parallel/DryRun: exit code = -1, wanted = 78: signal: killed #6364)
• Reduces start latency by ~1s if the user has already selected a driver
• Reduces start complexity

The only negative is that if the user has already specifically chosen a driver, we no longer show the alternatives possible.

UI change

Old:

✨ Automatically selected the 'hyperkit' driver (alternates: [virtualbox vmwarefusion docker])

New:

✨ Automatically selected the hyperkit driver. Other choices: virtualbox, vmwarefusion, docker (experimental)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tstromberg

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[tstromberg]

/ok-to-test

[minikube-pr-bot]

All Times minikube: [ 97.757348 97.367846 99.023391]
All Times Minikube (PR 6503): [ 95.805546 95.752421 97.685391]

Average minikube: 98.049528
Average Minikube (PR 6503): 96.414453

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6503) |
+----------------------+-----------+--------------------+
| minikube v           |  0.331484 |           0.191947 |
| Creating kvm2        | 20.202903 |          19.878649 |
| Preparing Kubernetes | 49.760967 |          49.793520 |
| Pulling images       |  3.670698 |           3.628442 |
| Launching Kubernetes | 20.013350 |          19.667306 |
| Waiting for cluster  |  1.405134 |           1.222890 |
+----------------------+-----------+--------------------+

[sharifelgamal]

This change LGTM, just needs to fix a lint error:

pkg/minikube/cluster/start.go:189: File is not `goimports`-ed (goimports)
  if driver.BareMetal(h.Driver.DriverName()) {

[minikube-pr-bot]

All Times minikube: [ 98.342826 99.801661 95.513690]
All Times Minikube (PR 6503): [ 97.358634 95.628702 98.990637]

Average minikube: 97.886059
Average Minikube (PR 6503): 97.325991

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6503) |
+----------------------+-----------+--------------------+
| minikube v           |  0.320863 |           0.195026 |
| Creating kvm2        | 19.813383 |          20.058561 |
| Preparing Kubernetes | 50.225235 |          50.922649 |
| Pulling images       |  4.193392 |           3.423215 |
| Launching Kubernetes | 20.095195 |          19.831940 |
| Waiting for cluster  |  1.224473 |           1.057923 |
+----------------------+-----------+--------------------+