Fix --reuse-db and --create-db not working together. Fixes #411

[ryan-shaw]

Improves upon #476 which fixes #411

Also fixes the test to include db exist check.

[codecov-io]

Codecov Report

Merging #560 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage      92%   92.02%   +0.01%     
==========================================
  Files          33       33              
  Lines        1627     1631       +4     
  Branches      140      140              
==========================================
+ Hits         1497     1501       +4     
  Misses         93       93              
  Partials       37       37

Flag	Coverage Δ
#dj110	84.79% <100%> (+0.03%)	⬆️
#dj111	86.08% <100%> (+0.03%)	⬆️
#dj18	85.65% <100%> (+0.03%)	⬆️
#dj19	84.67% <100%> (+0.03%)	⬆️
#dj20	84.36% <100%> (+0.03%)	⬆️
#djmaster	84.36% <100%> (+0.03%)	⬆️
#mysql_innodb	84.3% <100%> (+0.03%)	⬆️
#mysql_myisam	84.24% <100%> (+0.03%)	⬆️
#postgres	87.67% <100%> (+0.03%)	⬆️
#py27	89.39% <100%> (+0.02%)	⬆️
#py34	84.67% <100%> (+0.03%)	⬆️
#py35	84.79% <100%> (+0.03%)	⬆️
#py36	84.85% <100%> (+0.03%)	⬆️
#sqlite	86.14% <100%> (+0.03%)	⬆️
#sqlite_file	84.67% <100%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
tests/test_db_setup.py	100% <100%> (ø)	⬆️
pytest_django/fixtures.py	97.43% <100%> (+0.03%)	⬆️
pytest_django/plugin.py	86.32% <100%> (+0.04%)	⬆️

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[blueyed]

Merged in ac7eb3b, thanks.
Slightly adjusted/formatted the docs.