move neuvector ns definition to namespace-defs unit

Closes #1914 (closed)

#1914 (closed) is caused by the fact that the new neuvector-policies component added in kyverno-policies by !2721 (merged) is resulting in trying the creation of a Policy in neuvector namespace before this namespace is created (which would happen later, on reconciliation of neuvector-init unit).

This MR moves the creation of the namespace to the namespace-defs unit, which we introduced precisely to avoid this kind of dependency issues.