short circuit docker image creation if image with same tag already ex…

[danabens]

If the docker image for integration tests has already been created (with the same tag) don't attempt to recreate it.

tests/integ/test_experiment.py .......                                                                                                                                                             [  7%]
tests/integ/test_tracker.py .......                                                                                                                                                                [ 14%]
tests/integ/test_trial.py .....                                                                                                                                                                    [ 20%]
tests/integ/test_trial_component.py ....                                                                                                                                                           [ 24%]
tests/integ-jobs/test_track_from_processing_job.py s                                                                                                                                               [ 25%]
tests/integ-jobs/test_track_from_training_job.py s                                                                                                                                                 [ 26%]
tests/unit/test_base_types.py ...........                                                                                                                                                          [ 37%]
tests/unit/test_environment.py .....                                                                                                                                                               [ 43%]
tests/unit/test_experiment.py ............                                                                                                                                                         [ 55%]
tests/unit/test_metrics.py .........                                                                                                                                                               [ 65%]
tests/unit/test_tracker.py ...................                                                                                                                                                     [ 85%]
tests/unit/test_trial.py ........                                                                                                                                                                  [ 93%]
tests/unit/test_trial_component.py ......                                                                                                                                                          [100%]

================================================================================= 93 passed, 2 skipped in 41.33 seconds ==================================================================================
py36 run-test: commands[1] | coverage report --fail-under=80
Name                                                                     Stmts   Miss  Cover
--------------------------------------------------------------------------------------------
.tox/py36/lib/python3.6/site-packages/smexperiments/__init__.py              0      0   100%
.tox/py36/lib/python3.6/site-packages/smexperiments/_base_types.py          69      4    94%
.tox/py36/lib/python3.6/site-packages/smexperiments/_boto_functions.py      34      0   100%
.tox/py36/lib/python3.6/site-packages/smexperiments/_environment.py         36      1    97%
.tox/py36/lib/python3.6/site-packages/smexperiments/_utils.py               36     12    67%
.tox/py36/lib/python3.6/site-packages/smexperiments/api_types.py            91      5    95%
.tox/py36/lib/python3.6/site-packages/smexperiments/experiment.py           30      0   100%
.tox/py36/lib/python3.6/site-packages/smexperiments/metrics.py              76      4    95%
.tox/py36/lib/python3.6/site-packages/smexperiments/tracker.py             110      8    93%
.tox/py36/lib/python3.6/site-packages/smexperiments/trial.py                45      4    91%
.tox/py36/lib/python3.6/site-packages/smexperiments/trial_component.py      41      1    98%
--------------------------------------------------------------------------------------------
TOTAL                                                                      568     39    93%
________________________________________________________________________________________________ summary _________________________________________________________________________________________________
  py36: commands succeeded
  congratulations :)

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