lldpd: implement lldp_policy parameter

lldpd: implement lldp_policy parameter

For certain lldp_class scenarios (2 & 3) a policy must be set also.
Class 4 is default, although it's good to handle the policy eventuality.

Here, set a default lldp_policy for all lldp_class scenarios. Any
lldp_policy can now be set.

Depends on PR #14584 (which introduced an `if` block)

Tested on 22.03.5, 22.03.6

Signed-off-by: Paul Donald <newtwen@gmail.com>