make it easier to replace git.repo path's for airgap settings

Current process lends itself to manually replacing each and every *.git.repo, we should make this easier with tpl'ing

initial thoughts:

repo: https://repo1.dso.mil/platform-one/big-bang

istio:
  git:
    repo: {{ tpl .Values.repo . }}/apps/core/istio-controlplane.git

it would also make sense for our repos.tar.gz to also be structured that way, so a user can simply change .Values.repo