feat(sgw): add storagegateway_fault_tolerance check

[sansns]

Context

Checks to see if SGW gateways are running on EC2

Description

If the gateways are running on EC2, this introduces a single point of failure for any application relying on the SGW for a file storage

Checklist

• Are there new checks included in this PR? Yes
  • If so, do we need to update permissions for the provider? Please review this carefully. SecurityAudit contains storagegateway:List*
• Review if the code is being covered by tests.
• Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Attention: Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.88%. Comparing base (8a6e222) to head (4b90eb3).
Report is 165 commits behind head on master.

Files with missing lines	Patch %	Lines
.../services/storagegateway/storagegateway_service.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5570      +/-   ##
==========================================
+ Coverage   89.78%   89.88%   +0.09%     
==========================================
  Files        1090     1133      +43     
  Lines       33767    35312    +1545     
==========================================
+ Hits        30318    31739    +1421     
- Misses       3449     3573     +124     

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[MrCloudSec]

Hi @sansns, I have made several changes on the check. Let me know what you think!

[madereddy]

@MrCloudSec Changes look great.