Do not tear down database when run with --reuse-db. Fixes #411

[dekkers]

This will only teardown the database when --reuse-db is not used. It restores the behaviour from before the database fixture refactor and will make it possible to reuse a database with initial test data.

[blueyed]

@dekkers
Can you add a test for this, please?
Let us know if you need help with that.

[codecov-io]

Codecov Report

Merging #476 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #476   +/-   ##
=======================================
  Coverage   85.74%   85.74%           
=======================================
  Files          32       32           
  Lines        1614     1614           
  Branches      140      140           
=======================================
  Hits         1384     1384           
  Misses        177      177           
  Partials       53       53

Flag	Coverage Δ
#django110	81.72% <0%> (ø)	⬆️
#django111	83.45% <0%> (ø)	⬆️
#django17	79.61% <0%> (ø)	⬆️
#django18	80.79% <0%> (ø)	⬆️
#django19	79.86% <0%> (ø)	⬆️
#mysql_innodb	78.99% <0%> (ø)	⬆️
#mysql_myisam	78.93% <0%> (ø)	⬆️
#postgres	81.22% <0%> (ø)	⬆️
#py27	83.02% <0%> (ø)	⬆️
#py34	80.91% <0%> (ø)	⬆️
#py35	79.12% <0%> (ø)	⬆️
#py36	80.91% <0%> (ø)	⬆️
#pypy	81.59% <0%> (ø)	⬆️
#pypy3	80.79% <0%> (ø)	⬆️
#pytest3	85.74% <0%> (ø)	⬆️
#sqlite	81.59% <0%> (ø)	⬆️
#sqlite_file	80.29% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pytest_django/fixtures.py	83.54% <0%> (ø)	⬆️

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[blueyed]

Closing in favor of #560.