Refactor netns.execute so that it is not necessary to check namespace

I saw some code in a couple of reviews today that check whether a
namespace is set and run it under "ip netns exec ..." if it is.
Otherwise, it runs the command without it in the default namespace.

Change-Id: I55e8f4f3523ec7a7c5a6f082addf918952a05741
Closes-Bug: #1287524
Merge "Refactor netns.execute so that it is not necessary to check namespace"