At the moment, if you have a helmfile.yaml like so:
```
releases:
- name: metrics-server
namespace: kube-system
chart: stable/metrics-server
```
If you try to run `helmfile deps`, you will get a 0 exit code and no log
output at whatsoever, signaling that there weren't any problems, but no
lock file will get created.
For example:
```
root@316073d4a104:/# helmfile deps
root@316073d4a104:/#
```
This behavior doesn't appear to be documented and is unintuitive to the
user.
This change adds a warning output for this same use case:
```
root@316073d4a104:/# helmfile deps
There are no repositories defined in your helmfile.yaml.
This means helmfile cannot update your dependencies or create a lock file.
See https://github.com/roboll/helmfile/issues/878 for more information.
root@316073d4a104:/#
```
Fixes#878
921f69bae7