Honor DB username as set in env file

[etj]

Ref #11513
Use the DATABASE_URL if it exists, otherwise create it by also using the username.

Checklist

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[giohappy]

@etj to be done also for geonode-project

[codecov]

Codecov Report

Merging #11522 (1e03cc9) into master (80e2bbf) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11522      +/-   ##
==========================================
- Coverage   62.05%   62.05%   -0.01%     
==========================================
  Files         867      867              
  Lines       51724    51724              
  Branches     6469     6469              
==========================================
- Hits        32098    32095       -3     
- Misses      18086    18088       +2     
- Partials     1540     1541       +1