fix random test_task unittest fail due to os.environ mock

fix random test_task unittest fail due to os.environ mock

sometime Jenkins fails due to what I guess are concurrency problems
in os.environ mock

======================================================================
FAIL: tests.unit.benchmark.core.test_task.TaskTestCase.test_parse_suite_with_constraint_no_args
tags: worker-10
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.5/unittest/mock.py", line 1157, in patched
    return func(*args, **keywargs)
  File "/home/jenkins/opnfv/slave_root/workspace/yardstick-verify-euphrates/tests/unit/benchmark/core/test_task.py", line 208, in test_parse_suite_with_constraint_no_args
    task_files, task_args, task_args_fnames = t.parse_suite()
  File "/home/jenkins/opnfv/slave_root/workspace/yardstick-verify-euphrates/yardstick/benchmark/core/task.py", line 455, in parse_suite
    cur_pod = os.environ.get('NODE_NAME', None)
  File "/usr/lib/python3.5/unittest/mock.py", line 917, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/lib/python3.5/unittest/mock.py", line 976, in _mock_call
    result = next(effect)
StopIteration
Ran 1262 tests in 2.375s
FAILED (id=0, failures=1)
error: testr failed (1)
+ testr failing

Replace the mock decorator with a context manager to try to
reduce the scope and duration of the mock.

Change-Id: I342fe6c403e66c53ac4c39fd88fa9047cdfae5d9
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
(cherry picked from commit 2fadf7aec9e2761c39d29d8af1ee7d69d154652d)