This becomes a dictionary of key <lister-name>, value a dict of key
<instance-name>, value list of dict username/password.
This allows configuration change to allow the following:
```
credentials:
github: # github lister
github: # has only one instance (so far)
- username: some
password: somekey
- username: one
password: onekey
- ...
gitlab: # gitlab lister
riseup: # has many instances
- username: someone
password: ...
- ...
gitlab:
- username: someone
password: ...
- ...
```
Related T1772