Mix BZR_CONCURRENCY and --concurrency so both are available.

* bzrlib/tests/test_osutils.py:
(TestConcurrency.setUp): Simplified.
(TestConcurrency.test_local_concurrency_environment_variable):
Restore the env varible tests.
(TestConcurrency.test_option_concurrency): Test the global option.

* bzrlib/tests/__init__.py:
(TestCase._cleanEnvironment): Add BZR_CONCURRENCY.

* bzrlib/osutils.py:
(local_concurrency): Simplify implementation: environment variable
overrides OS specific definition.

* bzrlib/help_topics/__init__.py:
(_env_variables): Restore the documentation.

* bzrlib/commands.py:
(run_bzr): Simplify --concurrency handling: it just overrides the
environment variable.