Optimise the function.dep_attrs() function
Optimise the function.dep_attrs() function

Due to unfortunate copy-pasta, the original implementation of
function.dep_attrs() iterates over items in a dictionary instead of values.
Dictionary keys cannot have dependencies - they must be hashable and
Function objects are not. So for each dictionary key we call dep_attrs()
recursively 3 times as many times as we need to (for the key/value tuple,
the key, and the value). For template snippets that are dominated by
dictionaries, this means it took three times as long and used three times
as much memory and twice as much stack space as it needed to.

Change-Id: I13781540483daf88202d221a9f517746eebf0346
Partial-Bug: #1684272

noironetworks/heat,noironetworks/heat,openstack/heat,openstack/heat