As a user of ROSA UI, the UI should only offer me lists of instance types that are both

• available according to OCM (only instance types that we've opened to use with ROSA)
• available in the region I've selected in the UI
  This convergence of options is what I am offered in the ROSA CLI, so that I don't try to create a cluster in a region with an instance type for which there are none available to (not a question of quota, but a question of if the instance type is actually offered in a specific region).

The instance types offered via OCM are also frequently being updated, so they need to be sorted by instance type series, and the defaults should not be affected by the changing list of types offered. (the m5 series is always default and should remain default)