Following up on OCMUI-2460 , this ticket is about the same work but in day 2.

With max 500 nodes support for ROSA HCP, and up to 249 in OSD, it is not user friendly to have a dropdown menu with so many items. To get to the high values users will have to scroll a lot. We should move from dropdown menu to numerical text input, similar to the input fields when autoscaling is enabled. 

In the scope of this work, `buildOptions` helper function (https://github.com/RedHatInsights/uhc-portal/blob/master/src/components/clusters/common/machinePools/utils.ts#L262) which was originally designed to return an array of possible drop down options, should be refactored to return just the max number of nodes, and also renamed since "options" will not be relevant anymore.