Description of problem:

The ** value of the public subnet drop-down set as empty from the Hypershift ROSA wizard's networking > configuration step. The public subnet id drop-down is a required field but the user could navigate the next step without having a subnet value selected. This is wrong.

How reproducible:

 Always

Steps to reproduce:

1. Launch OCM Staging.
2. Open ROSA wizard and select control plane type as "Hosted"
3. Proceed to next steps by providing all required values in each step.
4. Reach to "Networking" step and see the Public subnet id drop-down value.
5. Do not choose a Public subnet id drop-down value(keep it empty) and click "Next" button.
6. See the behavior.

Actual results:
**

1. The public subnet id drop-down is always set as empty from the Hypershift ROSA wizard.
2. Although  public subnet id  is required field, the wizard allow to proceed to next step and this is wrong.

Expected results:

1. The public subnet id drop-down is always should not be  empty from the Hypershift ROSA wizard. Recommand to have representation "Select public subnet ID" indication in dropdown or keep a public subnet id value by default from the list. 
2. Throw validation error if  public subnet id  empty and user tries to move to next step.Next button should not allow to move to next step till valid public subnet supplied.

Refer screen recordings