Require configuring the NIC of VM as promiscuous (#243)

This change applies to working with a nested virtual environment. 

CloudStack's system VMs rely on the deployment VM for Internet access.
When the NIC of the deployment VM is not promiscuous, it will deny traffic from the system VMs, 
and thus, the system VMs cannot reach the Internet. This will create a lot of issues after CloudStack is installed.
It's better to explicitly mention this in the installation guide, so that people will use the correct 
setting from the beginning, instead of spending a lot of time to realize this is the problem.

Author: daweilics

source/quickinstallationguide/qig.rst

@@ -42,7 +42,10 @@ get you up and running with CloudStack with a minimum amount of trouble.
       You will need to have 1 NIC in your VM, bridged to the NIC of your laptop/desktop
       (wifi or wired NIC, doesn't matter), and optimally to set Adapter Type="Paravirtualized Network (virtio-net)"
       for somewhat better network performance (Settings of VM, Network section, Adapter1,
-      expand "Advanced"). Also, make sure you have allowed enough ram (6G+) and enough CPU cores (3+) for demo purposes.
+      expand "Advanced"). Make sure the NIC on your VM is configured as promiscuous (in VirtualBox, 
+      choose "Allow All" or just "Allow VMs" as the Promiscuous Mode), so that it can pass traffic from 
+      CloudStack's system VMs to the gateway. Also, make sure you have allowed enough ram (6G+) and 
+      enough CPU cores (3+) for demo purposes.
 
 
 High level overview of the process