Add Option to Override Shell

[pnovotnak]

I've added the option to override the shell in kustomization.yaml files through a new parameter generateShell which takes a list of strings. The first string is the binary to use as the alternate shell, all subsequent parameters are passed as arguments, ending with the command provided by the user in a generator.

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[monopole]

I'm wondering if this could be part of a struct (e.g. a GeneratorOptions struct, per #354 (comment)) sent to both the secret and configmap factory, even though at the moment the configmap generator doesn't use a shell.

I think it would make for a cleaner API to gather all generator inputs into one place, even if there's a difference at the moment between what the configmap and secret generators need.

WDYT?

[Liujingfang1]

I added generatorOptions in kusotmization type, which will provide overriding shell. #497