Add support for External platform to CSO

Epic Goal*

 
As defined in the  External platform enhancement , a new platform is being added to OpenShift. To accommodate the phase 2 work, the CSO should be updated, if necessary, to react to the "External" platform in the same manner as it would for platform "None".

Please see the  enhancement and the parent plan OCPBU-5 for more details about this process.

 
Why is this important? (mandatory)

In phase 2 (planned for 4.13 release) of the external platform enhancement, the new platform type will be added to the openshift/api packages. As part of staging the release of this new platform we will need to ensure that all operators react in a neutral way to the platform, as if it were a "None" platform to ensure the continued normal operation of OpenShift.

 
Scenarios (mandatory) 

Provide details for user scenarios including actions to be performed, platform specifications, and user personas.  

1. As a user I would like to enable the External platform so that I can supplement OpenShift with my own CSI driver. To ensure proper operation of OpenShift, the cluster storage operator should not react to the new platform or prevent my installation of the custom driver so that I can create clusters with my own topology.

 
Dependencies (internal and external) (mandatory)

This will require OCPCLOUD-1777

Contributing Teams(and contacts) (mandatory) 

Our expectation is that teams would modify the list below to fit the epic. Some epics may not need all the default groups but what is included here should accurately reflect who will be involved in delivering the epic.

• Development - mimccune@redhat.com , dmoiseev , Install Flex working group
• Documentation -
• QE - 
• PX - 
• Others -

Acceptance Criteria (optional)

Provide some (testable) examples of how we will know if we have achieved the epic goal.  

We are working to create an External platform test which will exercise this mechanism, see OCPCLOUD-1782

Drawbacks or Risk (optional)

Reasons we should consider NOT doing this such as: limited audience for the feature, feature will be superseded by other work that is planned, resulting feature will introduce substantial administrative complexity or user confusion, etc.

Done - Checklist (mandatory)

The following points apply to all epics and are what the OpenShift team believes are the minimum set of criteria that epics should meet for us to consider them potentially shippable. We request that epic owners modify this list to reflect the work to be completed in order to produce something that is potentially shippable.

• CI Testing - we will perform manual test while waiting for OCPCLOUD-1782
• Documentation - only developer docs need to be updated at this time
• QE - test scenario should be covered by a cluster-wide install with the new platform type
• Technical Enablement - n/a
• Engineering Stories Merged
• All associated work items with the Epic are closed
• Epic status should be “Release Pending”