properly iterate over IPs in check-vpc-nameservers.rb

[masneyb]

check-vpc-nameservers.rb would fail with the following error when
attempting to validate the VPC nameservers on the DHCP option set:

Check failed to run: undefined method `each_value' for
#<Aws::EC2::Types::DhcpConfiguration:0x00556320a7a318>

This corrects the iterator so that that sensu check works as expected.

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Is this in reference to an existing issue?

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• Update Changelog following the conventions laid out here

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Known Compatibility Issues

[majormoses]

@masneyb sorry for the delayed response, can you please rebase against master as #322 will fix the CI issue you are seeing.

[masneyb]

@majormoses: I created a new PR: #324 that is rebased off of master. I also added an entry to the CHANGELOG.