Allow empty configuration value

Some Nova configurations allows an empty value. In some case,
an empty string needs to be explicitly set otherwise
the default value is used. (such as dhcp_domain)

Change-Id: I410f981ec6976f9bb4e5877cc14730d17ff4225b
Rename nova_config rspec tests

The unit tests got updated when we changed nova_config provider
from parsed to ini_settings, but we forgot to rename the file.
Renaming it to better reflect which provider is tested.

Change-Id: Ib3c0581b589e01db804ffba54c7faf7f329ce844
Merge "Allow empty configuration value"