Expand documentation about on-prem storage requirements

The requirement on persistent volumes is clearly stated in https://access.redhat.com/documentation/en-us/red_hat_3scale/2.1/html-single/infrastructure/#system_requirements.

---- from the above section ----
Persistent Volumes:

3 RWO (ReadWriteOnce) persistent volumes for Redis and MySQL persistence
1 RWX (ReadWriteMany) persistent volumes for CMS and System-app Assets

I believe this section from official docs should be the goto place - not slides - for 3scale requirements. So any further clarifications should be in this section.

2) Is it clear that the requirement for RMX PVs implies a requirement for NPS, Gluster, AWS S3 etc?

No. The requirement is not obvious ( at least the first time around). It is covered in the OpenShift docs, but locating the OpenShift doc, and making the jump is not easy. So adding the following after the persistent volume requirements in the 3scale document section referenced above would help.

---- Expanded section in https://access.redhat.com/documentation/en-us/red_hat_3scale/2.1/html-single/infrastructure/#system_requirements

Persistent Volumes:

3 RWO (ReadWriteOnce) persistent volumes for Redis and MySQL persistence
1 RWX (ReadWriteMany) persistent volumes for CMS and System-app Assets

To select persistent volume that supports the required access modes, see Table 4.8 "Supported Access Modes for Persistent Volumes" in https://access.redhat.com/documentation/en-us/openshift_container_platform/3.6/html-single/architecture/#persistent-volumes.
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So to select storage, I can click on the table and the options for RMX would be NFS or GlusterFS . Note Slide mentions "AWS S3" but Table 4.8 references AWS EBS(Readonly) not AWS S3.

Another diff that needs to be reconciled: Slide has "- 1 RWX PV (amp.yml template) MUST be group writeable: chmod g+w" . But "group writeable:chmod g+w" requirement is not captured in https://access.redhat.com/documentation/en-us/red_hat_3scale/2.1/html-single/infrastructure/#system_requirements .