bfd: always export remote_state and remote_diagnostic to OVSDB

bfd: always export remote_state and remote_diagnostic to OVSDB

RFC 5880 specified bfd.RemoteSessionState as one of the state
variables.  In OVS implementation, this value is exported to OVSDB's
BFD status column of the interface table, as one of the map elements,
with the key of 'remote_state'.

It can be surprising when the 'remote_state' map element disappears
when BFD is in the 'DOWN' state, but otherwise always exported.
Change to always exporting it, to make it more predictable for
applications that monitors the BFD status column.

While at it, make the same change to 'remote_diagnostic', so that it
is also always exported to OVSDB for consistency.

VMWare-BZ: 1535979
Reported-by: Mihir Gangar <gangarm@vmware.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>