Don't use deprecated configparser.readfp

Running verify_tempest_config, it complains with:
  /opt/stack/tempest/tempest/cmd/verify_tempest_config.py:444:
  DeprecationWarning: This method will be removed in future versions.
  Use 'parser.read_file()' instead.

Use read_file.

Also, remove one extra use of six in this file.

Change-Id: Ia32df8e2be29bb0951efd08db9eed9e67d749e1f
1 file changed