In some cases, the NoCloud config drive is built using
`_` instead of `-` characters for the metadata and userdata paths.
This commit adds two configuration options to be able to set custom
values to those paths:
```ini
[nocloud]
metadata_file = "meta-data"
userdata_file = "user-data"
```
Fixes: https://github.com/cloudbase/cloudbase-init/issues/89
Change-Id: I312aa26ed9be6f22156ac238f456c3906d93760d
Signed-off-by: Adrian Vladu <avladu@cloudbasesolutions.com>