Improve README

Add examples on how to use tasks to manage virtual machines, install
satellite and nightly builds and how to specify a ssh key when running
fabric.
Merge pull request #34 from elyezer/improve-readme

Improve README
Add vm_list_base task

The vm_list_base task will list all available base images. Update README
to show how to use the vm_list_base task.

Also improve the vm_list to only show running virtual machines by
default.