feat(cli): allow export-roles to be beautified

[EBoisseauSierra]

Description

Expose json.dump(indent=*) to the fab export-roles --indent=* CLI command.

ADDITIONAL INFORMATION

• Has associated issue: Allow roles export to be beautified (pretty print) #1716
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1724 (ba7da50) into master (3a6b45b) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1724      +/-   ##
==========================================
+ Coverage   77.04%   77.06%   +0.01%     
==========================================
  Files          56       56              
  Lines        8219     8226       +7     
==========================================
+ Hits         6332     6339       +7     
  Misses       1887     1887              

Flag	Coverage Δ
python	77.06% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
flask_appbuilder/cli.py	58.37% <100.00%> (+1.36%)	⬆️
flask_appbuilder/security/manager.py	75.88% <100.00%> (ø)
flask_appbuilder/security/mongoengine/manager.py	66.15% <100.00%> (ø)
flask_appbuilder/security/sqla/manager.py	71.38% <100.00%> (ø)

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[EBoisseauSierra]

@dpgaspar : this is now ready for review!

FYI, the missing bits were:

• handling --indent=-1 “edge case” (as “-1” is interpreted as a string) — note that we might want to extract the try block in a dedicated method; I'd welcome your input there,
• declaring the new kwarg in all three flask_appbuilder/security/manager.py, flask_appbuilder/security/mongoengine/manager.py, and flask_appbuilder/security/sqla/manager.py,
• mock json.dumps (and not dump),
• assert one kwarg out of ≥1 is used.