Make kubectl gs template subcommand flags easier to understand

[marians]

Currently kubectl gs template cluster|nodepool --help show a lot of flags. This includes "global" kubectl flags which are not specific to the subcommand.

Many of the subcommand's flags are provider-specific. So to a certain use, many flags are not actually relevant.

We discussed options in SIG UX on Nov 30.

TODO

• (1) Hide global flags by default from help output, similar to what kubectl [command] --help does. (This applies to most kubectl-gs commands, not just template.)

• (2) Group the template subcommand flags into groups like "Common", "AWS specific", "Azure specific" etc.

• (3) Only show the relevant flag groups when the --provider flag is given. (Extension of (2)).