GenericCLI Fix: Commands set to 'false' in OnlyCmds are now correctly disabled

[izvyk]

Description

Closes #192

[Gerrit91]

Did you check all the dependents that it does not break anything?

[izvyk]

Yes, I've checked the dependents in metal-stack and metal-stack-cloud.

They all utilize the OnlyCmds helper function, like this:

// ...
OnlyCmds: genericcli.OnlyCmds(
    genericcli.ListCmd,
    genericcli.DescribeCmd,
),
// ...

This helper explicitly sets the map values to true.
From genericcli/cmds.go:

func OnlyCmds(cmds ...DefaultCmd) map[DefaultCmd]bool {
	res := map[DefaultCmd]bool{}

	for _, c := range cmds {
		res[c] = true
	}

	return res
}

Since all dependents already rely on this function to explicitly set true, this PR shouldn't cause any regressions.