This is important for deployments where the chart repo is protected by client cert validation.
helmfile.yaml is extended to support `certFile` and `keyFile`:
```
repositories:
- name: roboll
url: http://roboll.io/charts
certFile: optional_client_cert
keyFile: optional_client_key
```
Everything works the same if you don't provide values for them.
Regarding the implementation, the "helm repo add" command already supports the cert-file and key-file values, so we just have to pass them through.
- Simple copy-paste for the most part from sync job. I had started down
the path of adding in a meta PluginCommand directive and trying to make it
more modular, but in the end there are some small differences between
the execution that were a bit difficult to model and it just got ugly.
Figured keeping it simple would be easier to manage
Shane Starcher
Cedric Meury
Greg Burton
KUOKA Yusuke
Justin Nauman
rob boll