Added default SSH key support.

Added quite some time ago but forgot to commit. This adds a default
key setting on downburst so not only is the user's ssh key installed
but a 'generic' or 'default' ssh key (which you want on all your
machines) also gets installed. In our sepia lab case this is
teuthology's ssh key and the behavior can be disabled with a command
line arugment of --nokey to downburst.
Added support for creating guests of multiple architecture.

Downburst now supports creating guests that are x86_64/amd64 or i386.
This is done via either a commandline addition to the downburst
command eg. --arch=i386 or in the yaml file with arch: i386. The
default arch is x86_64/amd64. It will take both x86_64 or amd64 as
arguments to use a 64-bit image.