machine constraints

[William Reade]

Hi all

Machine constraints have now landed in the PPA, and the documentation --
at https://juju.ubuntu.com/docs/constraints.html -- should now be fully
up to date.

Briefly: you can now specify the hardware on which you want your EC2 
instances to run in reasonable detail, and you have some tools for
controlling deployment on orchestra and MAAS.

Please let us know if you encounter any problems!

Cheers
William