Don't create or check permissions on .plotly config directory until it a write operation is requested

[jonmmease]

Right now we always run the check to make sure read/write permissions are available for the user's .plotly directory. We do it with a call to tools.ensure_local_plotly_files() on the import of plotly/plotly/plotly.py. Becuase of our recursive imports this package is always imported and so we always check for these permissions.

Because of use-cases like that in #1068, where a bunch of jobs are running on a cluster and are just doing offline plotting, we shouldn't do this check unless we're actually using the plotly.plotly package to communicate with the plot.ly servers.

[jonmmease]

Done in #1195

[pb-cdunn]

I think this is still a problem.

_______ ERROR collecting tests/unit/test_pbreports_plot_plotly_helper.py _______
tests/unit/test_pbreports_plot_plotly_helper.py:7: in <module>
    from pbreports.plot import plotly_helper
pbreports/plot/plotly_helper.py:12: in <module>
    from plotly import graph_objs
/scratch/cdunn/repo/pbreports/build/lib/python2.7/site-packages/plotly/__init__.py:31: in <module>
    from plotly import (plotly, dashboard_objs, graph_objs, grid_objs, tools,
/scratch/cdunn/repo/pbreports/build/lib/python2.7/site-packages/plotly/plotly/__init__.py:10: in <module>
    from . plotly import (
/scratch/cdunn/repo/pbreports/build/lib/python2.7/site-packages/plotly/plotly/plotly.py:30: in <module>
    from plotly import exceptions, files, session, tools, utils
/scratch/cdunn/repo/pbreports/build/lib/python2.7/site-packages/plotly/files.py:41: in <module>
    _file_permissions = _permissions()
/scratch/cdunn/repo/pbreports/build/lib/python2.7/site-packages/plotly/files.py:28: in _permissions
    os.mkdir(TEST_DIR)
E   OSError: [Errno 17] File exists: '/home/UNIXHOME/cdunn/.test'

I had to re-raise in the except-catch block of _permissions in order to get this. from plotly import graph_objs seems to trigger it, and that's in the public API.

[pb-cdunn]

I want to make warnings into errors in my tests. (pytest -W error), so this is a big problem for me.