Change cpus config field to string to support max value

[spowelljr]

Fixes #15468

Before:

$ minikube config set cpus max

❌  Exiting due to MK_CONFIG_SET: set: strconv.Atoi: parsing "max": invalid syntax

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│    😿  If the above advice does not help, please let us know:                                                           │
│    👉  https://github.com/kubernetes/minikube/issues/new/choose                                                         │
│                                                                                                                         │
│    Please run `minikube logs --file=logs.txt` and attach logs.txt to the GitHub issue.                                  │
│    Please also attach the following file to the GitHub issue:                                                           │
│    - /var/folders/9l/6wpxv6wd1b901m1146r579wc00rqw3/T/minikube_config_15c3e99d63670bb33f9e3453813a4d46a56daa07_0.log    │
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After:

$ minikube config set cpus max
❗  These changes will take effect upon a minikube delete and then a minikube start

[k8s-ci-robot]

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This pull-request has been approved by: spowelljr

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