Feature: Mock region

[bblommers]

Feature to allow a user to specify a non-existing region. Having a non-existent region can be useful as an additional layer of protection, to ensure the user does not modify real AWS architecture.

This feature is disabled by default, but can be enabled with an environment variable. (MOTO_ALLOW_NONEXISTENT_REGION)

In order to make this work, all backends are now generated on-request, rather then on start-up.
This results in:

• A single place where we can decide whether a region is valid/allowed
• Better performance on start-up (we don't need to generate Backends for 27 regions, just the one required)
• Better performance on reset (as we only need to reset a single Backend, instead of all 27.)

Closes #4252

[codecov-commenter]

Codecov Report

Merging #4699 (d214b67) into master (691bb87) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4699      +/-   ##
==========================================
+ Coverage   95.28%   95.29%   +0.01%     
==========================================
  Files         556      556              
  Lines       61017    60594     -423     
==========================================
- Hits        58140    57745     -395     
+ Misses       2877     2849      -28     

Flag	Coverage Δ
servertests	47.49% <99.49%> (-0.33%)	⬇️
test_responses	32.08% <99.49%> (-0.42%)	⬇️
unittests	95.21% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
moto/applicationautoscaling/__init__.py	100.00% <ø> (ø)
moto/cloudwatch/__init__.py	100.00% <ø> (ø)
moto/cognitoidentity/__init__.py	100.00% <ø> (ø)
moto/datapipeline/__init__.py	100.00% <ø> (ø)
moto/elastictranscoder/__init__.py	100.00% <ø> (ø)
moto/emrcontainers/__init__.py	100.00% <ø> (ø)
moto/iot/__init__.py	100.00% <ø> (ø)
moto/iotdata/__init__.py	100.00% <ø> (ø)
moto/medialive/__init__.py	100.00% <ø> (ø)
moto/rds/__init__.py	100.00% <ø> (ø)
... and 86 more

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[bpandola]

I don't have the bandwidth to review all the changes across all 98 files, but I understand the problem this PR aims to solve and I think the implementation makes sense.

The added tests do seem to cover the new functionality, so if all the other tests still pass this LGTM.

[bblommers]

Understandable! Thanks @bpandola

[github-actions]

This is now part of moto >= 2.3.0.dev3