Add new disk_templates parameter to instance policy

This is a bit more complex patch, as it requires changing the
assumption that all keys in the policy dict points to values that are
themselves dicts. Right now we introduce an assumption that any
non-dicts are lists, we'll see in the future if this holds or whether
we need more complex type checking (manual, yay Python).

The patch also does some trivial style changes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>