Fix storage removal

[mtfranchetto]

Description

Fixes #286

Changes

Move from object to array for UI settings of a storage ("Use existing" toggle) to keep the storage config and the UI settings in sync (both use arrays now)

Changelog entry

Fix removing a shared storage

How Has This Been Tested?

• added 2-3 storages
• removed each of them in random order
• repeated the process a couple times

PR Quality Checklist

• [x] I added tests to new or existing code
• [ ] I removed hardcoded strings and used our i18n solution instead (see here)
• [x] I checked that npm run build builds without any error
• [x] I checked that clusters are listed correctly
• [x] I checked that a new cluster can be created (config is produced and dry run passes)
• [x] I checked that login and logout work as expected

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