[Improvement] Default template folder

[guidiego]

Resume

Problem: What happens if I don't provide a -t or -u?
Reply: Errors haha
Solution: Get templates from a default folder (set by the user using an env-var like DEFAULT_KUBE_RENDER_TEMPLATE_DIR)

Cases

• Local usage
  • I have a kube-templates' clone on my machine
  • I don't want / I forgot to use -u or -t
  • I want to set this repo as my "default" folder and simplify my compilation
• Cloudbuild usage
  • I want to use kube-render image to deploy my app
  • I want to make it fastier (without clone templates or configure git keys)
  • We could add kube-templates as submodules / default folder on it and make it easy
• Override case
  • I had some templates but I want to use some default one's
  • I could load it and complete with a second folder using -t or -u