make docker-push work with docker-tag artifacts

Ref. the docker builder documentation it should be possible to tag an artifact using
docker-tag and then pass it to docker-push using a set of sequential post-processors
e.g. 
```
  "post-processors": [
    [
      {
          "type": "docker-tag",
          "repository": "askholme/configurator",
          "tag": "newest"
      },
      "docker-push"
    ]
  ]
```
However in this gives below error, since docker-push refuses to deal with docker-tag artifacts.
```
==> docker: Running post-processor: docker-tag
    docker (docker-tag): Tagging image: 2626017c150ee05f6c30d2bde0c421aba19e9c4ffda4b93e1917e9b5a82887f4
    docker (docker-tag): Repository: askholme/nginx_media:latest
==> docker: Running post-processor: docker-push
Build 'docker' errored: 1 error(s) occurred:

* Post-processor failed: Unknown artifact type: packer.post-processor.docker-tag
Can only import from docker-import artifacts.
```
This PR updates docker-push to accept both docker-import and docker-tag artifacts
docker-tag itself checks for docker-import artifacts, so any unwelcome artifacts 
will still raise an error.