Setup remote desktop on Amazon EC2 Ubuntu 11.10

NX server is running over SSH, similar to sftp.
1. General steps (https://help.ubuntu.com/community/NomachineNX)
Note: when download nx deb packages, use the appropriate packages for your environment, the document assumes i386, but you may need 64 bits

2. NX remote desktop setup requires two types of users:
a). OS user, called "nx", for ssh login like normal ubuntu user, but the "nx" user's home is not at normal /home/nx, but at /usr/NX/home/nx
b). NX specific user. After you logged in via SSH, NX requires another authentication. So need to add nx user and enabled password authentication. (NX client downloaded from NoMachine seems only support password authentication for NX user. a little weird)

3. Setup OS "nx" user to have the public key for your ssh session. Test if you can ssh via "nx" user.
4. Enable EnablePasswordDB = "1" in server.cfg
5. Create an OS user first, then nxserver --adduser "username" since "The user must already exist on the host server when the command is executed." (from nxserver usage)
6. In NX client, load the private key for SSH session, and type "username" and password for NX user