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