util: add ensure_dir_exists parameter to write_file

[OddBloke]

This allows us to disable the ensure_dir call when it isn't
appropriate.

[smoser]

ensure_dir_exists -> mkdirs ?

2 things

• shorter
• plural - ensure_dir will create all parent directories on the way.

cons:

• isn't named 'ensure_dir' which lines up with the cloud-init function.

i'm fine with either.

[OddBloke]

Thanks for taking a look! I think I prefer ensure_dir_exists; it mirrors ensure_dir (and I would have named the parameter that if it didn't shadow the function in the body of write_files) so it's consistent within the codebase, and e.g. py.path uses ensure_dir for the same concept so I think it'll be natural even for people who don't know of the existence of util.ensure_dir.

[smoser]

good enough