Flush address during network restart(ifdown -a) for ubuntu

Ubuntu by default doesnt flush the addresses
Hence when a non-vlan interface gets tagged the already existing
addresses are not flushed from RT and they continue to get used

Fixing the same by calling flush during ifdown event

Change-Id: Ice0e6936f504348ae1c20c94eb42d158f5c96ed9
Merge "Flush address during network restart(ifdown -a) for ubuntu" into R2.0