Remove XML related code from RestClient

XML API tests have been removed, but there is still some code related
to XML API tests in RestClient class.
This patch remove it for code cleanup.

Change-Id: I098fe348220bb24c022c4b0753da322e1cdb1893
Raise a new exception NotImplemented for HTTP501

In _error_checker(), both HTTP500 and 501 are converted to the same
exception ServerFault. In addition, some method which extracts error
message raises a specific exception IdentityError without considering
HTTP code.
This patch adds a new exception NotImplemented and uses it for HTTP501
so that we can know which error response is returned from a server.

Change-Id: Ic8fc216377942619f11a2462b79d0597071ac294
Merge "Remove XML related code from RestClient"
Merge "Raise a new exception NotImplemented for HTTP501"