Replace custom Helm integration with PyHelm3

- `add_repo`: `Client.get_chart` (currently no authentication)
- `upgrade_install`: `Client.install_or_upgrade_release`
- `uninstall`: `Client.uninstall_release`
- `template`: `Client.template_resources`
- `get_manifest`: `Client.get_current_revision` → 
  - `ReleaseRevision.resources`
  - `ReleaseRevision.values`