ECR describe_image should return list of imageTags

[sthuber90]

Actual

Creating images put_image with the same image_manifest and different tags results in different images being created and returned through describe_image:

{
 "imageDetails": [
     {
         "imageSizeInBytes": 55600,
         "imageDigest": "sha256:abcdefg",
         "imageTags": [
             "v1"
         ],
         "registryId": "1234567890",
         "repositoryName": "test",
         "imagePushedAt": 1505819275.0
     },
     {
         "imageSizeInBytes": 55600,
         "imageDigest": "sha256:abcdefg",
         "imageTags": [
             "latest"
         ],
         "registryId": "1234567890",
         "repositoryName": "test",
         "imagePushedAt": 1508933240.0
     }
 ]
}

Expected

Creating and updating images with put_image with the same image_manifest should have the same image_digest and should be returned with a list of the corresponding tags through describe_image i.e.:

{
 "imageDetails": [
     {
         "imageSizeInBytes": 55600,
         "imageDigest": "sha256:abcdefg",
         "imageTags": [
             "v1",
             "latest"
         ],
         "registryId": "1234567890",
         "repositoryName": "test",
         "imagePushedAt": 1505819275.0
     }
 ]
}