Allow customization of API server instance group name

[nrb]

/kind feature

Describe the solution you'd like

Currently, CAPG creates an instance group named <infraID>-apiserver-<zone> for the API server machines. It would be helpful to allow customization of this name to enable migration from systems similar to CAPI onto CAPI itself.

The current value would remain the default, to keep compatibility, and because most users likely won't use this option.

Anything else you would like to add:

This request comes from OpenShift adopting Cluster API and moving away from its legacy Cluster API fork, Machine API. While this feature may impact few users, it's an important enhancement to allow adoption.

OpenShift currently names its API server instance group <infraID>-master-<zone>.

Also, this was briefly discussed on Slack as a reasonable enhancement.