Refactor serializer import to XmlBodyMiddleware

Previously, the keystone/middleware/core.py module would import
keystone/common/serializer.py regardless of the XmlBodyMiddleware class
being used. This change modifies keystone/common/serializer.py to raise
an error if there is an issue with using lxml at runtime.

Change-Id: I977275d0df8666b49ddbfadd4f5606d7c2292b1f
Closes-Bug: 1351016
Merge "Refactor serializer import to XmlBodyMiddleware"