_yaml.py: Only retrieve provenance in node_get() when needed

We were indiscriminately retrieving the node's provenance data in the
`node_get()` function which was accounting for approximately a third of
the total runtime of `node_get()` which dominates pre-scheduler time in
`bst build`.  This change ameliorates that situation by only retrieving
the provenance data when it's actually needed.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>