Fix hwvtep.yang

BFD configuration and parameters are defined as string. This is incorrect
and makes it difficult to use BFD cofigs and params. This fix creates
them as lists of key:value pairs of type string:string.

Change-Id: I8af37e443cc04d3665ef4c70824b02d71c14b9b5
Signed-off-by: Vishal Thapar <vishal.thapar@ericsson.com>