BUG/MINOR: stats: Fix color of draining servers on stats page

BUG/MINOR: stats: Fix color of draining servers on stats page

This patch fixes #53 where it was noticed that when an active
server is set to DRAIN it no longer has the color blue reflected
within the stats page. This patch addresses that and adds the
color back to drain. It's to be noted that backup servers are
configured to have an orange color when they are draining.

Should be backported as far as 1.7.

(cherry picked from commit b428517fee01b92546ee82cf420644d548f15a90)
Signed-off-by: Willy Tarreau <w@1wt.eu>