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/sshd.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+set -e
+source /bd_build/buildconfig
+set -x
+
+SSHD_BUILD_PATH=/bd_build/services/sshd
+
+## Install the SSH server.
+$minimal_apt_get_install openssh-server
+mkdir /var/run/sshd
+mkdir /etc/service/sshd
+touch /etc/service/sshd/down
+cp $SSHD_BUILD_PATH/sshd.runit /etc/service/sshd/run
+cp $SSHD_BUILD_PATH/sshd_config /etc/ssh/sshd_config
+cp $SSHD_BUILD_PATH/00_regen_ssh_host_keys.sh /etc/my_init.d/
+
+## Install default SSH key for root and app.
+mkdir -p /root/.ssh
+chmod 700 /root/.ssh
+chown root:root /root/.ssh
+cp $SSHD_BUILD_PATH/keys/insecure_key.pub /etc/insecure_key.pub
+cp $SSHD_BUILD_PATH/keys/insecure_key /etc/insecure_key
+chmod 644 /etc/insecure_key*
+chown root:root /etc/insecure_key*
+cp $SSHD_BUILD_PATH/enable_insecure_key /usr/sbin/