BZ2051452: Add subnet requirements to install procedures

[cherraz]

Added a reusable snippet across some of the install assemblies.

FIXES BZ: https://bugzilla.redhat.com/show_bug.cgi?id=2051452
Additional information presented by the person that opened the BZ : https://github.com/openshift/installer/blob/master/docs/user/metal/install_ipi.md

QUESTIONS:

• Does this apply to other install methods? I've only added it to "bare metal" and "vsphere" for now, which is what is mentioned in the BZ, but I wonder if the general statement may be valid for any installation with user-provisioned infrastructure.

Because of this, I've added it to the top-level section of bare metal and vsphere. This means it would apply to all bare metal install procedures (ipi or not) and same for vsphere procedures.

Is this the best place to add this information? Should there be an additional mention on bare-metal ipi just in case? Should it only apply to ipi?

List of install methods: https://deploy-preview-43386--osdocs.netlify.app/openshift-enterprise/latest/installing/installing-preparing.html

• Which versions does this apply to? 4.8 onwards?

PREVIEW:

• https://deploy-preview-43386--osdocs.netlify.app/openshift-enterprise/latest/installing/installing_bare_metal/preparing-to-install-on-bare-metal.html
• https://deploy-preview-43386--osdocs.netlify.app/openshift-enterprise/latest/installing/installing_vsphere/preparing-to-install-on-vsphere.html

VERSIONS: ???

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[cybertron]

This does not apply to baremetal. It's not a problem to install multiple clusters in a subnet on that platform in any supported release. The only ones where this is a concern are vsphere, openstack, and ovirt, and those should also be fixed in 4.11 by openshift/machine-config-operator#3016

Also note that there is a way to safely run multiple clusters on a subnet. The process described in https://access.redhat.com/solutions/6450581 needs to be followed to ensure there are no conflicts.

This does only apply to IPI, so we don't want to mention it in any UPI docs.

[cherraz]

Based on Ben Nemec's comment, this issue is fixed on 4.11 (main) so I'll fix it on 4.6 - 4.10 only. So, I'll close this PR and open a new one.