Enables users to override previously 'hidden' env vars.

* OpenCAFE expects several environment variables to be
  set before execution.  The runner wrappers in /drivers
  manage setting the env vars via the TestEnvManager.
  This change allows users to manually set the env vars
  that are usually set via the TestEnvManager through
  the runners, allowing users to bypass the manager
  and the engine.config file completely.

* Overrides are disabled by default, and enabled by
  setting 'CAFE_ALLOW_MANAGED_ENV_VAR_OVERRIDES' to any
  value in the environment.

* Reorganizes existing managed envrionment variables
  into a Namespace dictionary, 'MANAGED_VARS', for easy
  reference via the TestEnvManager.

* Fixed _NamespaceDict exception message grammar.

Change-Id: Ieae72dce995d1e19687b9daebd5c2e06503aaaae