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 for Essex.

Change-Id: I815b27ff2845293e3d6771ff8f99944ec08ccbd5