Move xml common code into the common dir

This commit moves tempest.services.compute.xml.common to the proper
location in tempest.common and renames the file to xml_utils.py. This
module was used in every client not just compute so the appropriate
place for this is in the common dir.

Change-Id: I42a8338fd507c1224fa9fd2cdc5c1b0700f7a26a
37 files changed