feat(dependencies): git dependencies allow submodules

[MatteoVoges]

Fixes #914

Proposed Changes

• Kapitan dependencies with type: git allow checkout with submodules now.
• e.g.

• ---
  parameters: 
    kapitan:
      dependencies:
         - type: git
           source: https://github.com/kapicorp/kapitan.git
           submodules: true 
           output_path: output/directory

  fetches kapicorp/kapitan with its submodule kapicorp/reclass

To Do

• Add documentation
• Add a test example

---

Contributed by

[MatteoVoges]

I added tests and documentation. I thought about enabling the flag by default, because it would make sense to initialize the submodules of a repository. What do you think? @ademariag @ramaro @uberspot @gburiola

Ready for review 🚀

[ramaro]

@MatteoVoges as discussed, let's have submodules enabled by default and allow disabling if necessary

[MatteoVoges]

Done.