Network: replace neutron check with decorator

Commit a581823ecee286c5c6329c9c373f0fea0b6b729f added in a
validator that would check if neutron was enabled.

It is much cleaner to do this as a decorator. The decorator
here needs to preserve the function signature.

Change-Id: I355e4bcee5e93a7f1459672e10ad7793d2bf8a49
Merge "Network: replace neutron check with decorator"