Description of problem:
- Default (openshift project) imageStream https://github.com/openshift/openshift-ansible/blob/release-3.6/roles/openshift_examples/files/examples/v3.6/image-streams/image-streams-rhel7.json creates the ImageStream with ImageStreamTag alias like [1].

- However, when we try to create such ImageStream(Tags) by `oc tag` command, it fails due to the validation error.

```
# oc tag registry.access.redhat.com/rhscl/perl-520-rhel7:latest perl:latest --source=docker
# oc tag perl:latest perl:5.20 --source=istag 
# oc tag 5.20 perl:latest --source=istag --alias
error: --source=ImageStreamTag requires a valid <name>:<tag> in SOURCE
```

Version-Release number of selected component (if applicable):
- OCP 3.3 (also reproduced by OCP 3.6)

How reproducible: 100%

Steps to Reproduce:
- 1. Add alias of ImageStreamTag with following commands:

```
# oc tag registry.access.redhat.com/rhscl/perl-520-rhel7:latest perl:latest --source=docker
# oc tag perl:latest perl:5.20 --source=istag 
# oc tag 5.20 perl:latest --source=istag --alias
```
Actual results:
- oc tag 5.20 perl:latest --source=istag --alias fails due to this error.

```
# oc tag 5.20 perl:latest --source=istag --alias
error: --source=ImageStreamTag requires a valid <name>:<tag> in SOURCE
```

Expected results:
- Create the ImageStream as [1].

Additional info: proposal patch https://github.com/openshift/origin/pull/17377

[1]
```
apiVersion: v1
kind: ImageStream
metadata:
  annotations:
    openshift.io/display-name: Perl
  name: perl
spec:
  tags:
  - annotations:
      version: "5.20"
    from:
      kind: DockerImage
      name: registry.access.redhat.com/rhscl/perl-520-rhel7:latest
    generation: 2
    importPolicy: {}
    name: "5.20"
  - annotations:
      version: "latest"
    from:
      kind: ImageStreamTag
      name: "5.20"
    generation: 1
    importPolicy: {}
    name: latest
```