Investigate use of save_logging, reset_logging and remove them from tests

[Jim Baker]

A number of tests (mostly in ensemble.hook, but also in ensemble.control) use the pattern of save_logging/reset_logging, however, this is not used in other tests.

At least one test is currently unreliable in trunk, due to a logging related issue, when looped repeatedly:

./test -u ensemble.hooks.tests.test_invoker
...
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1020, in _inlineCallbacks
    result = g.send(result)
  File "/home/jbaker/canonical/ensemble/mine/expose-hook-commands/ensemble/hooks/tests/test_invoker.py", line 399, in test_spawn_fail
    self.assertInDefaultLog("ERROR")
  File "/home/jbaker/canonical/ensemble/mine/expose-hook-commands/ensemble/lib/testing.py", line 136, in assertInDefaultLog
    return self.assertIn(expected_value, content)
twisted.trial.unittest.FailTest: 'ERROR' not in "hook:DEBUG:: hook fail-hook ended, exit code Traceback (most recent call last):\nFailure: ensemble.errors.FormulaInvocationError: Error processing '/home/jbaker/canonical/ensemble/mine/expose-hook-commands/ensemble/hooks/tests/hooks/fail-hook': exit code 1.\n.\n"

ensemble.hooks.tests.test_invoker.InvokerTest.test_spawn_fail
-------------------------------------------------------------------------------
Ran 23 tests in 11.353s

FAILED (failures=1, successes=22)

At standup, it was determined that this usage should be investigated, conclusions reported to the mailing list, and removed from code so as to be more consistent with other tests.