Made services installation optional during build

You can user `ENV` directive in Dockerfile to disable the installation
for some services or change `image/buildconfig`.

The flags are :

DISABLE_SSHD
DISABLE_CRON
DISABLE_SYSLOG

image/services/syslog-ng/syslog-ng.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+set -e
+source /bd_build/buildconfig
+set -x
+
+SYSLOG_NG_BUILD_PATH=/bd_build/services/syslog-ng
+
+## Install a syslog daemon.
+$minimal_apt_get_install syslog-ng-core
+mkdir /etc/service/syslog-ng
+cp $SYSLOG_NG_BUILD_PATH/syslog-ng.runit /etc/service/syslog-ng/run
+mkdir -p /var/lib/syslog-ng
+cp $SYSLOG_NG_BUILD_PATH/syslog_ng_default /etc/default/syslog-ng
+touch /var/log/syslog
+chmod u=rw,g=r,o= /var/log/syslog
+cp $SYSLOG_NG_BUILD_PATH/syslog-ng.conf /etc/syslog-ng/syslog-ng.conf
+
+## Install syslog to "docker logs" forwarder.
+mkdir /etc/service/syslog-forwarder
+cp $SYSLOG_NG_BUILD_PATH/syslog-forwarder.runit /etc/service/syslog-forwarder/run
+
+## Install logrotate.
+$minimal_apt_get_install logrotate
+cp $SYSLOG_NG_BUILD_PATH/logrotate_syslogng /etc/logrotate.d/syslog-ng