Clear L2 output config on interface mode change to L3 (VPP-651)

Clear L2 output config on interface mode change to L3 (VPP-651)

With VPP-651, the L2 output config with L2-tag rewrite was not
cleared when a sub-interface is deleted. Subsequently, when the
same sw_if_index was reused for another interface, the L2 output
config with L2-tag rewrite remained on the new interface.

On deleting a (sub-)interface which is in L2 mode, it will be
changed to L3 mode first to clear any L2 config. The L2 to L3 mode
change path did address L2 input config cleanup. It is now fixed
to also clear L2 output config.

Change-Id: I3352a89d92e1b27340a5adcf75bbaa01a5050c29
Signed-off-by: John Lo <loj@cisco.com>