Change boot order in virtualbox scripts

Master node will boot from disk/cdrom/net
Slave nodes will boot from net/disk
It is done in order to meet our requirements:
http://docs.mirantis.com/fuel/fuel-4.1/install-guide.html#fuel-installation-procedures

Partial-Bug: #1316583
Change-Id: I2bdd929e48cf5c4cc687b7ef77ea749cc8c1750e