Propagate files in nested stacks

Share files from the parent template in StackResource, to be able to
reference files in resources like AutoScalingGroup

Closes-Bug: #1294033
Change-Id: Id325690e4b1c42ef2b6463769da0ae99571e7a56
Order imports in alphabetical order (7/9)

This patch is one in a series to re-enable H306 style check rule
(imports are in alphabetical order). It touches roughly half of tests
(up to and including test_m*).

Implements: blueprint reduce-flake8-ignored-rules (partial)
Change-Id: Id1e10f7a519ceeb4339488760b36950440461edb
Merge "Propagate files in nested stacks"
Merge "Order imports in alphabetical order (7/9)"