promote docker driver priority from "experimental" to "fallback"

[medyagh]

This PR should be merged. as last PR before releasing 1.8.0
If tests on this PR look good at that time, we can announce docker driver GA.

it will still not be the prefered as "hyperkit" on mac or "hyperv" on window but it will be higher priority

closes #6791

[k8s-ci-robot]

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[medyagh]

/ok-to-test

[minikube-pr-bot]

All Times minikube: [ 90.126926 90.054317 92.855479]
All Times Minikube (PR 6791): [ 90.530727 91.367424 94.643487]

Average minikube: 91.012240
Average Minikube (PR 6791): 92.180546

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6791) |
+----------------------+-----------+--------------------+
| minikube v           |  0.316718 |           0.371059 |
| Creating kvm2        | 19.613462 |          20.022210 |
| Preparing Kubernetes | 49.599565 |          49.908691 |
| Pulling images       |           |                    |
| Launching Kubernetes | 20.203802 |          20.511263 |
| Waiting for cluster  |  0.061667 |           0.060694 |
+----------------------+-----------+--------------------+