Remote registry - Githubissues

Allow the ability to sign images the formats of ImageStreamTag, ImageStreamImage, and DockerImage and include the ability to add pull secrets for remote registries.

Reproduce Tests:

Follow the Workflow Openshift example in the README to test the ImageStreamTag repository type. Make sure to use the following command when testing as described in the development.md document instead of applying the operator.yaml file as described in the README for the OpenShift Workflow examle.
```
$ operator-sdk run --local --namespace="image-management" 
```
For the ImageStreamImage I grabbed the SHA256 digest from the DockerImageReference attribute in the ImageStreamTag that is created in the dotnet-example Workflow in the README.

apiVersion: imagesigningrequests.cop.redhat.com/v1alpha1
kind: ImageSigningRequest
metadata:
  name: dotnet-app
spec:
  containerImage:
    kind: ImageStreamImage
    name: dotnet-example@sha256:763683295a9cc2b0e03ae7a415fce417fca5388935ba74e2b5d9d4e9b6ca6178

Used the following to test ContainerRepository

apiVersion: imagesigningrequests.cop.redhat.com/v1alpha1
kind: ImageSigningRequest
metadata:
  name: dotnet-app
spec:
  containerImage:
    kind: ContainerRepository
    name: quay.io/redhat-cop/image-scanning-signing-service@sha256:a47ae897b964f1e543452c31a24bbd3d46ed5830f4a6d9992be97d0ce61ceb6b

and

apiVersion: imagesigningrequests.cop.redhat.com/v1alpha1
kind: ImageSigningRequest
metadata:
  name: dotnet-app
spec:
  containerImage:
    kind: ContainerRepository
    name: quay.io/redhat-cop/image-scanning-signing-service:latest

For a private repository I made one of my quay.io images private and added a pull secret

apiVersion: imagesigningrequests.cop.redhat.com/v1alpha1
kind: ImageSigningRequest
metadata:
  name: dotnet-app
spec:
  containerImage:
    kind: ContainerRepository
    name: quay.io/cnuland/hello-openshift:latest
  pullSecret:
    name: quay

NOTE: The E2E image now points by default to the signing image built by this repo. It now can run a test for both ImageStreamTag and ContainerRepository with tag. To run the E2E tests you'll need to make two modifications since,

name: SIGN_SCAN_IMAGE
  value: "quay.io/cnuland/image-signing-centos8"

and

image: quay.io/cnuland/image-signing-operator

Both pointing to the latest versions of the signer and operator.

redhat-cop / image-scanning-signing-service

Remote registry #29