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 Folsom.

Change-Id: I6a4051b5e66f3ce5a330b2589c42e6e9e5b9268e
Merge "Add a safe_minidom_parse_string function." into stable/folsom