Check allNamespace config value while loading configuration file

[silvin-lubecki]

- How to verify it

# Add allNamespaces to your config file
$ cat ~/.docker/config.json
{
    "kubernetes": {
        "allNamespaces": "disabled"
    }
}
# Try some command
$ docker version
...
# Amend your config file with unknown value
$ cat ~/.docker/config.json
{
    "kubernetes": {
        "allNamespaces": "unknown"
    }
}
# Now a warning is displayed
$ docker version
WARNING: Error loading config file: /Users/silvin/.docker/config.json: invalid 'kubernetes.allNamespaces' value, should be 'enabled' or 'disabled': unknown
...

- Description for the changelog

• Restrict kubernetes.allNamespaces value to 'enabled' or 'disabled' in configuration file

- A picture of a cute animal (not mandatory but encouraged)

[silvin-lubecki]

Maybe we need a json schema on the configuration file... WDYT @vdemeester @thaJeztah ?

[vdemeester]

LGTM 🎋
For json-schema, meh, we might at some point but it would probably be overkill for now 😓

[thaJeztah]

Agreed that JSON-schema could be nice at some point (also related to #1058), but I think there's more enhancements to be made (multiple configurations you can switch between etc), so perhaps better to keep it for a future rewrite of this stuff

[thaJeztah]

LGTM, thanks!