Fix argument parsing bugs by using dedicated Viper instances

[t0yv0]

This stopped working as expected, instead generating a --generic style version:

pulumictl get version --language python

The first bad commit is f5ca64e and last good commit is 2218df7.

It turns out the issue is with viper arg parsing. Introducing a new "github.com/pulumi/pulumictl/cmd/pulumictl/convert-version" module with a new command reused the global viper instance. So this got called twice in different commands:

	viper.BindPFlag("language", command.Flags().Lookup("language"))

And it broke. Basically the new convert-version command broke the old get version command.

This is rather alarming, so the fix introduces dedicated viper state into every sub-command to avoid this collision in the future.

[t0yv0]

Err, this might not be quite right. githubToken = viper.GetString("token") may fail to work in sub-commands that don't bind it correctly to the local viper instance anymore. Some of this sharing was intentional?

[t0yv0]

Appended a few fixes, this should work now. When doing viper.GetString("token") most subcommands imply the "token" that was bound in main.go so this lookup should still use global viper state; for most of the variables they're bound locally and should use local viper.New().