[AIRFLOW-2407] Declare as global virtualenv_string_args

[cclauss]

Declare as global virtualenv_string_args defined elsewhere

Without this change flake8 will complain of undefined name virtualenv_string_args which is discussed at:

• https://github.com/apache/incubator-airflow/blob/master/airflow/operators/python_operator.py#L174

flake8 testing of https://github.com/apache/incubator-airflow on Python 3.6.3

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./tests/operators/test_virtualenv_operator.py:166:19: F821 undefined name 'virtualenv_string_args'
            print(virtualenv_string_args)
                  ^
./tests/operators/test_virtualenv_operator.py:167:16: F821 undefined name 'virtualenv_string_args'
            if virtualenv_string_args[0] != virtualenv_string_args[2]:
               ^
./tests/operators/test_virtualenv_operator.py:167:45: F821 undefined name 'virtualenv_string_args'
            if virtualenv_string_args[0] != virtualenv_string_args[2]:
                                            ^

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-XXX
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a JIRA issue.

Description

• Here are some details about my PR, including screenshots of any UI changes:

• See above.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  It fixes tests flake8 that currently fail.

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | __python3 -m__ flake8 --diff

[codecov-io]

Codecov Report

Merging #3293 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3293      +/-   ##
==========================================
- Coverage   75.89%   75.88%   -0.01%     
==========================================
  Files         197      197              
  Lines       14706    14706              
==========================================
- Hits        11161    11160       -1     
- Misses       3545     3546       +1

Impacted Files	Coverage Δ
airflow/models.py	86.64% <0%> (-0.05%)	⬇️

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[cclauss]

Closed in favor of #3307