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/logrotate_syslogng

@@ -0,0 +1,38 @@
+/var/log/syslog
+{
+	rotate 7
+	daily
+	missingok
+	notifempty
+	delaycompress
+	compress
+	postrotate
+		sv reload syslog-ng > /dev/null
+	endscript
+}
+
+/var/log/mail.info
+/var/log/mail.warn
+/var/log/mail.err
+/var/log/mail.log
+/var/log/daemon.log
+/var/log/kern.log
+/var/log/auth.log
+/var/log/user.log
+/var/log/lpr.log
+/var/log/cron.log
+/var/log/debug
+/var/log/messages
+{
+	rotate 4
+	weekly
+	missingok
+	notifempty
+	compress
+	delaycompress
+	sharedscripts
+	postrotate
+		sv reload syslog-ng > /dev/null
+		sv restart cron-log-forwarder > /dev/null
+	endscript
+}