Improve concurrent support for AWS services

[Aboisier]

This PR aims to address the TODOs left during the refactoring (#183).

[codecov-io]

Codecov Report

Merging #288 into refactoring/resource-configs will increase coverage by 0.09%.
The diff coverage is 27.27%.

@@                       Coverage Diff                        @@
##           refactoring/resource-configs     #288      +/-   ##
================================================================
+ Coverage                         33.66%   33.75%   +0.09%     
================================================================
  Files                               161      161              
  Lines                              5686     5706      +20     
================================================================
+ Hits                               1914     1926      +12     
- Misses                             3772     3780       +8

Impacted Files	Coverage Δ
ScoutSuite/providers/aws/resources/ec2/vpcs.py	100% <ø> (ø)	⬆️
...tSuite/providers/aws/resources/elasticache/vpcs.py	85.71% <ø> (-3.18%)	⬇️
...coutSuite/providers/aws/resources/rds/snapshots.py	21.05% <ø> (-3.95%)	⬇️
...iders/aws/resources/elasticache/parametergroups.py	40% <ø> (ø)	⬆️
ScoutSuite/providers/aws/facade/facade.py	44% <ø> (-0.74%)	⬇️
...ite/providers/aws/resources/rds/parametergroups.py	35.71% <ø> (ø)	⬆️
ScoutSuite/providers/aws/facade/basefacade.py	75% <ø> (ø)	⬆️
...tSuite/providers/aws/resources/rds/subnetgroups.py	40% <ø> (-5.46%)	⬇️
ScoutSuite/providers/aws/resources/regions.py	38.09% <ø> (-2.82%)	⬇️
ScoutSuite/providers/aws/resources/s3/service.py	50% <ø> (-2.39%)	⬇️
... and 39 more

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[misg]

In the end, this PR mainly aims to improve concurrent support for all the fetching done for AWS services. This is done mainly by refactoring the code so to use the new function get_and_set_concurrently that easily enable to fetch and set concurrently all attributes related to a specific kind of resource.

[Aboisier]

Approved, @misg !