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/cron/cron.runit

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec /usr/sbin/cron -f

image/services/cron/cron.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+set -e
+source /bd_build/buildconfig
+set -x
+
+$minimal_apt_get_install cron
+mkdir /etc/service/cron
+chmod 600 /etc/crontab
+cp /bd_build/services/cron/cron.runit /etc/service/cron/run
+
+## Remove useless cron entries.
+# Checks for lost+found and scans for mtab.
+rm -f /etc/cron.daily/standard
+rm -f /etc/cron.daily/upstart
+rm -f /etc/cron.daily/dpkg
+rm -f /etc/cron.daily/password
+rm -f /etc/cron.weekly/fstrim