Set promote_secondaries when creating namespaces

there is a sysconf entry which controls how deletion of the primary ip
is performed (/proc/sys/net/ipv4/conf/all/promote_secondaries). when set
instead of deleting the secondary addresses, one of them will be
promoted to primary ip.

Without it, when init_l3 called on a port, it may unexpectedly delete
some useful ips.

Change-Id: I0b1b3bd6ade21129532c842daa31059ea164719e
Closes-bug: 1343320
Merge "Set promote_secondaries when creating namespaces"