Allow ETSI VNF without other node template

In an attempt is made to onboard and import a csar to the ETSI SOL v2.5.1 model which does not contain a node template other than the VNF node template then an exceptio is thrown. 

The ETSI SOL001 spec indicates that such a template is required, however for a CNF this often will not be applicable. There is no reason in SDC to enforce this (as the functionality will work perfectly fine as long as it is expecting that this can be optional). Therefore we will remove this restriction in order to make SDC more flexible to the needs of CNF packages