tunnels: Don't require remote endpoint or ID during creation.

Before lightweight tunnels existed, it really didn't make sense to
create a tunnel that was not fully specified, such as without a
destination IP address - the resulting packets would go nowhere.
However, with lightweight tunnels, the opposite is true - it doesn't
make sense to require this information when it will be provided later
on by the route. This loosens the requirements for this information.

An alternative would be to allow the relaxed version only when
COLLECT_METADATA is enabled. However, since there are several
variations on this theme (such as NBMA tunnels in GRE), just dropping
the restrictions seems the most consistent across tunnels and with
the existing configuration.

CC: John Linville <linville@tuxdriver.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>