Add a 'build' subcommand

[degremont]

Similar to what docker does and one of the reason docker is so successful, please a 'build' subcommand for PCOCC enabling a way to:

1. Start a VM
2. Configure its content (using cloud-config)
3. Automatically detects end of configuration
4. Stop VM
5. Save its image

This would be very helpful feature and combine with #2, #3 and #4 would able shareable PCOCC config to create images (likewise Dockerfile)

[ypsah]

Step 3 can be implemented by polling on "/var/lib/cloud/instance/boot-finished" which is created on a VM once cloud-init completes.