use proxy mode on vxlan interface only when l2-population is activated

the proxy mode was set by default on every vxlan interfaces
which leads to inaccessibility between VM that are not hosted
on the same host in a vxlan network

Closes-Bug: #1237082

Change-Id: I34028ee0bdfdccda61c6a29f58759259da060b68
(cherry picked from commit 345f8e1fdc65e0cf23369488d4318c0755d6b65d)