Update order of hostname detection logic

The merge of https://github.com/jenkinsci/datadog-plugin/pull/346 did not take into account the changes from https://github.com/jenkinsci/datadog-plugin/pull/345 causing build failures. This PR refactors the getAWSInstanceId to use the new HTTP client

Additionally changes the order of the hostname collection so that the AWS instance param takes precedence over the HOSTNAME variable

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jenkinsci / datadog-plugin