oc-mirror should pull the graph-data YAML tarball from OSUS (phase 1)

Note:

This epic is a follow up to https://issues.redhat.com/browse/OTA-226 . Initially we want to fix this in OTA-255 but later we pivoted to this epic as this is a cleaner solution and will need less maintenance from the team. Also it will complement the current oc-mirror workflow.

Context:

As an administrator, I would like to keep the graphs from the local instance OpenShift update service (OSUS) in the disconnected environment same as public instance of OSUS run by Red Hat.

As part of the current method to deploy the OpenShift Update Service (OSUS) operator in disconnected/ restricted network environments customers need to build the graph data container image themselves [1] . We went with release of the OSUS without releasing the graph-data image because ACM needed OSUS. However asking customers to build an image to use with OSUS operator is not an acceptable solution for long term.

Customers are waiting for OTA-255 epic to get fixed (it is kind of blocking a deal) as the current solution does not work for them. Mostly because the current workflow (using oc-mirror) involves (as described below) goes to cincinnati-graph-data github repository to get the content for the init graph-data container which the OpenShift update service needs to serve upgrade graphs in disconnected environments.

• OC-mirror  has automated the workflow to mirror release payload to the disconnected environment. It also copies the content of cincinnati-graph-data then builds the cincinnati-graph-data container during the process.
• However it needs files from https://github.com/openshift/cincinnati-graph-data to build the graph-data container image.

This is not acceptable for the customer as they do not trust github content and only want content from Red Hat for the disconnected environment.

We have discussed the topic with the technical account managers working with the US department of energy and  Daniel Messer as he is the product manager for oc-mirror. Here is the feedback from the discussions.

• Customers like the oc-mirror workflow. As it has automated the hard part of running clusters in disconnected environments. However they do not like it when  it reaches out to public websites like github which they do not trust. 
• They do not have any opinion about whether the graph data image needs to be built and signed by Red Hat or oc-mirror builts it.
• As long as we get stuff from Red Hat it is fine with them.

Goal:

• oc-mirror downloading the graph-data tarball from OSUS  API endpoint.