fix unittest test_deploy_try_retrieve_context_does_not_exist

fix unittest test_deploy_try_retrieve_context_does_not_exist

unittest is failing due to mock being called multiple times.

I assume this is due to parallelization of tests in Jenkins somehow

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FAIL: test_deploy_try_retrieve_context_does_not_exist (benchmark.contexts.test_heat.HeatContextTestCase)
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Traceback (most recent call last):
  File "/home/jenkins/opnfv/slave_root/workspace/yardstick-verify-master/.tox/py3/lib/python3.5/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/home/jenkins/opnfv/slave_root/workspace/yardstick-verify-master/yardstick/tests/unit/benchmark/contexts/test_heat.py", line 309, in test_deploy_try_retrieve_context_does_not_exist
    mock_path_exists.assert_called_once_with(key_filename)
  File "/home/jenkins/opnfv/slave_root/workspace/yardstick-verify-master/.tox/py3/lib/python3.5/site-packages/mock/mock.py", line 947, in assert_called_once_with
    raise AssertionError(msg)
AssertionError: Expected 'exists' to be called once. Called 3 times.

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Change-Id: I696c63ef9f7943b841e2d3d533c92f18aa7044da
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>