Description of problem:
The custom partitioning spoke doesn't work with the disks selected in the storage spoke if the disk selection is changed and the custom partitioning spoke is re-visited.

Version-Release number of selected component (if applicable):
Fedora-Server-netinst-x86_64-Rawhide-20200502.n.0.iso
anaconda-33.12-1.fc33

How reproducible:
Always

Steps to Reproduce:
1. Start graphical installation on a system with 2 or more disks
2. Proceed to the storage spoke
3. Select the first drive (vda) and go to custom partitioning
4. Return back to the storage spoke
5. Select only the second drive (vdb) and go back to custom partitioning
6. Let anaconda create mount points

Actual results:
The custom storage spoke says "0 storage devices selected".
Newly created mount points are on the first disk (vda).

Expected results:
The custom storage spoke says "1 storage device selected".
Newly created mount points are on the second disk (vdb).