BUG/MINOR: servers: Don't make "server" in a frontend fatal.

When parsing the configuration, if "server", "default-server" or
"server-template" are found in a frontend, we first warn that it will be
ignored, only to be considered a fatal error later. Be true to our word, and
just ignore it.

This should be backported to 1.8 and 1.7.
diff --git a/src/server.c b/src/server.c
index d96edc7..4498fd8 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1937,7 +1937,7 @@
 			goto out;
 		}
 		else if (warnifnotcap(curproxy, PR_CAP_BE, file, linenum, args[0], NULL))
-			err_code |= ERR_ALERT | ERR_FATAL;
+			err_code |= ERR_WARN;
 
 		/* There is no mandatory first arguments for default server. */
 		if (srv) {