[MINOR] externalize the "balance" option parser to backend.c

A new function "backend_parse_balance" has been created in backend.c,
which is dedicated to the parsing of the "balance" keyword. It will
provide easier methods for adding new algorithms.
diff --git a/include/proto/backend.h b/include/proto/backend.h
index d70c68f..fc71717 100644
--- a/include/proto/backend.h
+++ b/include/proto/backend.h
@@ -36,6 +36,8 @@
 int srv_count_retry_down(struct session *t, int conn_err);
 int srv_retryable_connect(struct session *t);
 int srv_redispatch_connect(struct session *t);
+int backend_parse_balance(const char **args, char *err,
+			  int errlen, struct proxy *curproxy);
 
 void recount_servers(struct proxy *px);
 void recalc_server_map(struct proxy *px);