MEDIUM/BUG: Only explicitly report "DOWN (agent)" if the agent health is zero

MEDIUM/BUG: Only explicitly report "DOWN (agent)" if the agent health is zero

Make check check used to report explicitly report "DOWN (agent)" slightly
more restrictive such that it only triggers if the agent health is zero.

This avoids the following problem.

1. Backend is started disabled, agent check is is enabled
2. Backend is stabled using set server vip/rip state ready
3. Health is marked as down using set server vip/rip health down

   At this point the http stats page will report "DOWN (agent)"
   but the backend being down has nothing to do with the agent check

This problem appears to have been introduced by cf2924bc2537bb08c
("MEDIUM: stats: report down caused by agent prior to reporting up").

Note that "DOWN (agent)" may also be reported by a more generic conditional
which immediately follows the code changed by this patch.

Reported-by: Mark Brooks <mark@loadbalancer.org>
Signed-off-by: Simon Horman <horms@verge.net.au>