Description of problem:

For ROSA Hypershift, the machine pool is always created only on specific private subnet id irrespective of private subnet id chosen in "Add machine pool" dialog under machine pool tab.
After few investigations, it has identified that,  whenever user creates a new machine pool , it always picked private subnet id belongs to public subnet id's AZ used in cluster creation. It didn't respect the user's choice on private subnet id from "add machine pool" dialog. This is wrong.

Please review the screen recordings MPAddingBehaviorD2.mp4

Steps to reproduce:

1. Launch OCM UI staging.
2. Create a ROSA hypershift cluster with multiple machine pools belongs different AZ (Note the AZ of public subnet id used in the installation , ex us-west-2a).  
3. Open  ROSA Hypershift cluster.
4. Go to "Machine pool" tab.
5. Click "Add machine pool" button.
6. Select a private subnet id (ex: from us-west-2a AZ) , fill all required values and create machine pool.
7. View the private subnet id and AZ for newly added machine pool.
8. Repeat the step 4 to 6 with private subnet ids from all supported AZ (ex:us-west-2c, us-west-2b)

Actual results:

In step 6, The newly created machine pools are  created under the private subnet Id belongs to cluster's public subnet AZ (in this case us-west-2a) used during installation. Because of this situation, as part of Day 2 operation i.e. user is unable to create new machine pools with private subnet ids from all supported AZ(ex: in this case us-west-2c, us-west-2b AZ ) and user selection from "Add machine pool" dialog never respected. This is wrong and create confusions.

Expected results:

User should be able to create machine pools with private subnets from all supported AZ listed under "add machine pool " dialog.