node_filters should be available on requirement definition in addition to requirement assignment.

In the current spec grammar we can use the node_filters to specify how a requirement should be fulfilled automatically by a TOSCA orchestrator.

node_filters can be specified only on a requirement assignment (meaning only on a node template and not a node type).

For some TOSCA usage which implies reusable types some users of the spec want to create (node) types that includes some script and let other users reuse these types by referencing them in their templates.

In such situations being able to specify node_filters on a requirement can bring a lot of value (for example if the script is linux related I need to specify in the requirement definition that I need a container with a node filter to specify that the os capability of the node must supports linux).

I think that this can also be used in other situations where I want to express a requirement on multiple capabilities (as node filter allows to add more than one capability name/type).