Support for EC2 instance roles when pushing to ECR

[sithmein]

It's already possible to push images to AWS ECR using access and secret key. In AWS EC2 it's possible to attach a role to the instance that grants this instance the appropriate permissions to push to ECR. Using the instance metadata service, you can get temporary access and secret keys that you can use in subsequent requests. Therefore instead of having to provide the keys as "plain text" to Maven you could determine them automatically. The process is straight forward:

• curl -i http://169.254.169.254/latest/meta-data/iam/security-credentials returns the instance's role
• curl -i http://169.254.169.254/latest/meta-data/iam/security-credentials/<role> returns the temporary credentials:

{
  "Code" : "Success",
  "LastUpdated" : "2019-03-21T13:47:55Z",
  "Type" : "AWS-HMAC",
  "AccessKeyId" : "ABCDEFG...",
  "SecretAccessKey" : "+r6wb...",
  "Expiration" : "2019-03-21T20:05:53Z"
}

I'm happy to work on a PR for this but would first ask for opinions on how this should be configured. An easy approach would be to try getting the instance credentials if the registry matches the ECR pattern but no username and password (access/secret key) are provided. This would not require any extra configuration.

[rhuss]

Closing at the PR is already merged (thanks !). Going to make a release over the weekend, so you can expect support for this soon.