Support multiple --mount-string arguments

[mans0954]

When run with the docker driver, minikube takes a --mount-string argument. Provided the minikube container does not yet exist, it is created with the specified host folder or docker volume mounted at the specified location e.g.

minikube start --mount  --mount-string "host_volume:/mount_point"

Mounts the host_volume docker volume at /mount_point in the minikube container.

In fact the --mount-string flag can be specified multiple times, but only the last specified is used.

A very small change to cmd/minikube/cmd/start_flags.go would allow multiple --mount-string arguments to be used.

[mans0954]

/assign mans0954

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/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues and PRs.

This bot triages issues and PRs according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
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/lifecycle rotten

[spowelljr]

One thing I'm not sure of is whether 9P supports multiple string arguments out of box or if we're going to have to loop over all the mount strings. Doesn't matter either way, but just something to think about.

[mans0954]

I sketched out a solution to this in #12734 - I still think it would be a useful feature, but haven't had time to finish it myself.