vtep: additions to BFD configuration and status reporting

This commit adds default values for some BFD configuration keys
(bfd_config_local:bfd_dst_mac and bfd_params:enable). It also adds new
BFD status keys (bfd_enabled and bfd_info).

Signed-off-by: Ariel Tubaltsev <atubaltsev@vmware.com>
Signed-off-by: Bruce Davie <bdavie@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>