Adds tests to cover Swift's crossdomain middleware

It adds one test for now, a single unidentified GET call to
"/crossdomain.xml" and compares the response's body to the expected XML
file.

The test can be disabled if marked explicitely as such in the
tempest.conf file

Change-Id: Id88e9da59288f118a605bf20769378190aabdeca
Merge "Adds tests to cover Swift's crossdomain middleware"