Lets start using the new BFD yang model and translate the BFD session
configuration commands to use the northbound.
One important change: all sessions will default to use
`VRF_DEFAULT_NAME` (usually "default") when no VRF is configured. All
places which search for BFD sessions must now take this into account.
Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>
