bfd: schema: added a schema for project's BFD data.

bfd-session element is linked with bgp-router via bfd-session-enabled link,
making BGP session monitoring possible.

Change-Id: I4e97403dd3a43327695b7f823902689c0cb0fcf6
Signed-off-by: Mateusz Zalega <mateusz.zalega@codilime.com>
Merge "bfd: schema: added a schema for project's BFD data."