Island: Use MONGO_URL instead of hard-coded values for flask-security

[cakekoa]

What does this PR do?

Addresses the issue by parsing MONGO_URL into its constituent parts and using them to set the values provided to flask-security.

Issue #4170.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Do all unit tests pass?
• Do all end-to-end tests pass?
• Any other testing performed?

  Tested by running monkey-island and mongo in a docker-compose script, setting the MONKEY_MONGO_URL using the mongo service name

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.07%. Comparing base (9324b32) to head (e7c59c7).
Report is 1 commits behind head on develop.

❗ Current head e7c59c7 differs from pull request most recent head 3e7073f. Consider uploading reports for the commit 3e7073f to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4172      +/-   ##
===========================================
- Coverage    77.07%   77.07%   -0.01%     
===========================================
  Files          444      444              
  Lines        14136    14138       +2     
  Branches        18       18              
===========================================
+ Hits         10896    10897       +1     
- Misses        3240     3241       +1     

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