Add a safe_minidom_parse_string function.

Adds a new utils.safe_minidom_parse_string function and
updates external API facing Nova modules to use it.
This ensures we have safe defaults on our incoming API XML parsing.

Internally safe_minidom_parse_string uses a ProtectedExpatParser
class to disable DTDs and entities from being parsed when using
minidom.

Fixes LP Bug #1100282.

Change-Id: Ib90d6379320ff1d007f8a661f7ddaa286ba6918e