Detect incompatible DJANGO_SETTINGS_MODULE

[yakky]

if DJANGO_SETTINGS_MODULE is set in a way which is incompatible with django CMS installer, the install will fail with obscure errors

By checking on start, we can ensure to return a clearer error message to the user

Fix #330

[codecov]

Codecov Report

Merging #338 into develop will increase coverage by 0.22%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #338      +/-   ##
===========================================
+ Coverage     90.7%   90.92%   +0.22%     
===========================================
  Files            7        7              
  Lines          656      661       +5     
  Branches       139      140       +1     
===========================================
+ Hits           595      601       +6     
  Misses          28       28              
+ Partials        33       32       -1

Impacted Files	Coverage Δ
djangocms_installer/config/__init__.py	91.01% <100%> (+0.83%)	⬆️

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[coveralls]

Coverage increased (+0.03%) to 95.783% when pulling cdaf72f on feature/detect_settings_module_env into 85238dd on develop.