[Storage] Fix method used to get the AWS domain for FSx.

In particular, now it does not need anymore to access the undefined
node attribute in US ISO regions.

Signed-off-by: Giacomo Marciani <mgiacomo@amazon.com>

cookbooks/aws-parallelcluster-environment/libraries/fsx.rb

@@ -1,4 +1,10 @@
 def aws_domain_for_fsx(region)
   # DNS names have the default AWS domain (amazonaws.com) also in China and GovCloud.
-  region.start_with?("us-iso") ? aws_domain : CLASSIC_AWS_DOMAIN
+  if region.start_with?("us-iso-")
+    US_ISO_AWS_DOMAIN
+  elsif region.start_with?("us-isob-")
+    US_ISOB_AWS_DOMAIN
+  else
+    CLASSIC_AWS_DOMAIN
+  end
 end

cookbooks/aws-parallelcluster-environment/spec/unit/libraries/fsx_spec.rb

@@ -0,0 +1,30 @@
+require 'spec_helper'
+
+describe 'aws-parallelcluster-environment:libraries:aws_domain_for_fsx' do
+  shared_examples 'a valid aws_domain_for_fsx function' do |region, expected_aws_domain|
+    it 'returns the correct AWS domain' do
+      result = aws_domain_for_fsx(region)
+      expect(result).to eq(expected_aws_domain)
+    end
+  end
+
+  context 'when in US-ISO region' do
+    include_examples 'a valid aws_domain_for_fsx function', 'us-iso-WHATEVER', 'c2s.ic.gov'
+  end
+
+  context 'when in US-ISOB region' do
+    include_examples 'a valid aws_domain_for_fsx function', 'us-isob-', 'sc2s.sgov.gov'
+  end
+
+  context 'when in CN region' do
+    include_examples 'a valid aws_domain_for_fsx function', 'cn-WHATEVER', 'amazonaws.com'
+  end
+
+  context 'when in GovCloud region' do
+    include_examples 'a valid aws_domain_for_fsx function', 'us-gov-WHATEVER', 'amazonaws.com'
+  end
+
+  context 'when in whatever else region' do
+    include_examples 'a valid aws_domain_for_fsx function', 'WHATEVER-ELSE', 'amazonaws.com'
+  end
+end