Tap: Fix hardware address assignment

Tap: Fix hardware address assignment

[VPP-200]: Previously, VPP set the same hardware address to tap interface as of
Linux side tap interface, if neither hardware address nor 'random'
keyword were given explicitly on VPP command line during tap creation or
modification, which was default case.

This patch makes sure to set the different hardware addresses randomly
as default case. While one can set unique hardware address or similar to
one at Linux side using VPP tap command line.

Change-Id: Ie0a82f3706834e87426d66c6e869ec4edfefe932
Signed-off-by: Mohsin KAZMI <sykazmi@cisco.com>