pin knife-rackspace to 0.9.0 or higher

we require --ssh-keypair to be available, which was introduced in
[v0.9.0](https://github.com/opscode/knife-rackspace/blob/master/CHANGELOG.md#v090).
Adds private networking for Digital Ocean

Enables private networking for new droplets if the region supports it.
Fails gracefully if not: https://developers.digitalocean.com/droplets/.

> private_networking Optional, Boolean, enables a private network interface if the region supports private networking
Updates dop version to support private networking and scrubbing
Use sudo to create symlink to dsh. #596
make small improvements to the rackspace script

knife-rackspace does not respect the --ssh-identity option,
so we have to deal with the default SSH key i.e. id_rsa.pub.
The server name is also randomized so duplicate controllers
can exist. The default user is root.
remove redundant prepare script

rackspace only relies on one image, not two
use standardized format for naming nodes/layers
append node id to end of name
renamed prepare script
added nova client fo uploading SSH keys
update Rackspace readme docs
bash script should source rackspacerc
update prepare script reference
pin knife-rackspace to 0.9.0 or higher

we require --ssh-keypair to be available, which was introduced in
[v0.9.0](https://github.com/opscode/knife-rackspace/blob/master/CHANGELOG.md#v090).
make small improvements to the rackspace script

knife-rackspace does not respect the --ssh-identity option,
so we have to deal with the default SSH key i.e. id_rsa.pub.
The server name is also randomized so duplicate controllers
can exist. The default user is root.
remove redundant prepare script

rackspace only relies on one image, not two
use standardized format for naming nodes/layers
append node id to end of name
renamed prepare script
added nova client fo uploading SSH keys