Add NodeType requirements for PNF Descriptor chapter

Proposed Change:

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5.1.10.8. Node Types

Requirement: R-535009

target: PNF

keyword: MUST

introduced:  Dublin

The PNFD provided by a PNF vendor MUST comply with the following Node Types as specified in ETSI NFV-SOL001 standard :

— tosca.nodes.nfv.PNF

— tosca.nodes.nfv.PnfExtCp

— tosca.nodes.nfv.C

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