change nova_config to use ini_file

This commit refactors the nova_config type to use
the ini_file provider. This will ensure that this type
is consistent with other types.

The usage of inifile from nova_config is slightly difference
from other similar types. It does not require a section name,
and assumes that all sections are in the default section.

This is for backwards compatibility with the other types
and will likely change in the next major version.

this commit also updates nova_paste_init to support purging.