dpctl: cleaner dpctl output for tunnel ports.

dpctl: cleaner dpctl output for tunnel ports.

Currently dont-fragment and TTL are initialized to zero, but
those are not default config for tunnel ports.  dpctl
does not show default config of a port.  So by setting these
values to default we can get cleaner `dpctl show` output.

% ovs-dpctl show
system@ovs-system:
	port 0: ovs-system (internal)
	port 1: br0 (internal)
	port 4: gre_sys (gre: df_default=false, ttl=0)

% ovs-dpctl show # After initializing default values.
system@ovs-system:
	port 0: ovs-system (internal)
	port 1: br0 (internal)
	port 4: gre_sys (gre)

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>