Environment Variable RELEASE_UPRADER_ALLOW_THIRD_PARTY spelling is wrong

[Stephan Rügamer]

Think this is an old spelling error.

Fixing this might help others.
Error is in DistUpgrade/DistUpgradeController.py:

  # skip mirror check if special environment is set
        # (useful for server admins with internal repos)
        if (self.config.getWithDefault("Sources","AllowThirdParty",False) or
            "RELEASE_UPGRADER_ALLOW_THIRD_PARTY" in os.environ):
            logging.warning("mirror check skipped, *overriden* via config")
            mirror_check=False

[Brian Murray (brian-murray)]

 $ bzr log -r2965
------------------------------------------------------------
revno: 2965
committer: Mathieu Trudel-Lapierre <email address hidden>
branch nick: ubuntu-release-upgrader
timestamp: Tue 2016-03-29 23:19:22 -0400
message:
  DistUpgrade/DistUpgradeController.py: fix typo in variable name for
  RELEASE_UPGRADER_ALLOW_THIRD_PARTY; this will help running with third-
  party mirrors in sources via just the environment variable for one-shot
  deals.