MTU: set interface mtu tap

MTU: set interface mtu tap

This patch introduces standard command line 'set interface mtu' for setting the MTU
for tap devices. It allows user to set the mtu in between 68 to 65535 bytes
aligned to what linux kernel supports for tun/tap devices.

Right now tapcli set the same MTU size for each tap interface. But it should be set
and configure to per interface rather than per tap instance.

Change-Id: I81b7f3ad95ca56d585907ff8f51d9935a428e01b
Signed-off-by: Mohsin Kazmi <sykazmi@cisco.com>