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/sshd/00_regen_ssh_host_keys.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -e
+if [[ ! -e /etc/service/sshd/down && ! -e /etc/ssh/ssh_host_rsa_key ]] || [[ "$1" == "-f" ]]; then
+	echo "No SSH host key available. Generating one..."
+	export LC_ALL=C
+	export DEBIAN_FRONTEND=noninteractive
+	dpkg-reconfigure openssh-server
+fi