fix: add IAM endpoint for us-isob-*

[trivikr]

Confirmed that region is set for IAM endpoint using AWS REPL

aws-sdk> let client = new AWS.IAM({ "region": "us-isob-east-1" });

aws-sdk> let req = client.listRoles();

aws-sdk> req.httpRequest.region
'us-isob-east-1'

Checklist

• npm run test passes
• changelog is added, npm run add-change

[codecov-io]

Codecov Report

Merging #3056 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3056   +/-   ##
=======================================
  Coverage   97.05%   97.05%           
=======================================
  Files         330      330           
  Lines        9521     9521           
  Branches     1747     1747           
=======================================
  Hits         9241     9241           
  Misses        280      280

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