some fixes for the nova cloud adapter

Using the NeCTAR Australian Research Cloud (OpenStack) I found these issues.

1. the `private` key does not appear, so I made it optional.

2. the `region_name` here is `"Melbourne"`, ie. mixed case. I suggest whoever has uppercase requirements can specify the `region_name` as uppercase in the provider settings, or `get_entry` is modified to be case insensitive.