avoid race in elan bc group update

avoid race in elan bc group update

elan broad cast group update happens from
1) first elan dpn interface add (runs in oper default shard leader node)
2) l2gw connection add (runs in eos/cluster singleton owner node)

To avoid the race between these two
add the l2gw tep to operational elan external teps and keep the job key as
elan name

handle the tunnel ip change of l2gw gracefully

Change-Id: I2e195dc2c35e75521b31c2a0f358b2034b99874e
Signed-off-by: K.V Suneelu Verma <k.v.suneelu.verma@ericsson.com>