Improve TH local variables naming

This patch addresses two issues with our TH code:

- using non-unique names (e.g. "std" for a local name, instead of
  "std_XXXX" random names), which can leads to conflicts; on the other
  hand, this makes the generated code a bit harder to parse
- since only a few Python/JSON names have dashes in them, we didn't
  handle those, resulting in variables named like "disk-templates",
  which is not good; we now handle it the same as '_', i.e. we use it
  as a breaker for camel-casing

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>