Add ability to specify arbitrary key value pairs to be used by driver when launching instance

The use case I would like to support is the passing of a kernel command line at instance launch time. The driver for docker could use this metadata to pass environment variables it needs. Some discussion around this took place on the mailing list: http://lists.openstack.org/pipermail/openstack-dev/2013-December/022363.html
That discussion started with: http://lists.openstack.org/pipermail/openstack-dev/2013-December/022346.html

With glance images we have a way to associate arbitrary metadata attributes with the image. I could see using this mechanism to associate some default set of environment variables.

eg use a 'env_' prefix for glance image attributes

We've got a couple of cases now where we want to overrides these same things on a per-instance basis. Kernel command line args is one other example. Other hardware overrides like disk/net device types are another possibility

Rather than invent new extensions for each, we should have a way to pass arbitrary attributes along with the boot
API call, that a driver would handle in much the same way as they do for glance image properties. Basically think of it as
a way to custom any image property per instance created.