Use root_helper when checking namespace existence

Use the root helper in the ip netns list command executed by the IP library
when checking for the existence of a namespace.  This prevents an unprivileged
l3 agent from erroneously trying to create another namespace when one already
exists.

Closes-Bug: #1348812
Change-Id: If7560161de3be6066af0d9866e6b5cd7c7247c33
(cherry picked from commit 6d2ded2e42239a2ea3590bc1a3b02623b34952a1)