Fix per unit storage throughput

[mtfranchetto]

Description

Fixes the PerUnitStorageThroughput on Lustre FSx not being added to the final cluster configuration, starting from an empty template.
Closes #274.

Changes

• put PerUnitStorageThroughput in the configuration when a FSx Lustre is created
• extracted PerUnitStorageThroughput default values into their own constants

Changelog entry

• Fixes PerUnitStorageThroughput on Lustre FSx not being added to the final cluster configuration

How Has This Been Tested?

• Started a new wizard, adding a FSx Lustre filesystem: the PerUnitStorageThroughput parameter is added to the configuration
• When changing the value of PerUnitStorageThroughput it is reflected in the configuration

PR Quality Checklist

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

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