Adds the --test-repo flag to cafe-runner to override default_test_repo

 * Passing any test repo package name to --test-repo allows the runner
   to target that test repo instead of the one defined in
   the engine.config file (this flag does not modify the
   engine.config file).  The flag will also override the
   CAFE_OPENCAFE_ENGINE_default_test_repo environment variable.

 * Adds support for test_repo overriding to the TestEnvManager,
   allowing support to be added to other runners fairly easily.

Change-Id: Idbabe6f97e5afb1fdcfa68b7e1a3f2ca7735a67b
Merge "Adds the --test-repo flag to cafe-runner to override default_test_repo"