Add support for Rackspace cloud

Add support Rackspace cloud into Vagrantfile using environment variables
so users don't have to modify the Vagrantfile to enter their credentials

Add documentation explaining the usage of environment variables and how
to setup environment to create a Rackspace Cloud VM.

Provide easy way of setting up required environment variables through
setupRackspace.sh.

Change-Id: I10dec3a47aa2fa5368097cce489ab6c43a31331e