Dynamic ETSI manifest naming during packaging

[mpeuster]

The first version always uses a fixed name for the ETSI manifest when a package is created.

This is still aligned to the ETSI spec but requires Entry-Manifest: etsi_manifest.mf to be present in the TOSCA.meta (which is the case in the current implementation)

We can improve this by also dynamically naming the *.mf like the main descriptor to be even more aligned.

This is not super important.