Update the default VXLAN destination UDP port to the IANA assigned port

VXLAN was recently assigned UDP port 4789 by IANA. This
comit updates the OVS VXLAN implementation to reflect the new UDP port
number.

Cc: Kenneth Duda <kduda@aristanetworks.com>
Signed-off-by: Kyle Mestery <kmestery@cisco.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Add FAQ entries around the VXLAN support in Open vSwitch.

Add a section to the FAQ explaining VXLAN with a pointer to the IETF draft.
Add sections detailing how much of the VXLAN protocol is currently supported
in OVS, along with a section explaining the default UDP port and how to change
this when creating VXLAN ports.

Signed-off-by: Kyle Mestery <kmestery@cisco.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>