Merge 5.3.1-dev-SOL002 in 5.3.1-dev

Giacomo Berninirequested to merge
5.3.1-dev-SOL002 into 5.3.1-dev

[version-upd][SOL002][VNF-LCM][v5.3.1][all-Test-IDs]Update version of NFV SOL specification from 5.2.1 to 5.3.1 in the reference field

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