In iwd.py make sure all the static methods that touch IWD storage take the
storage_dir parameter instead of hardcoding IWD_STORAGE_DIR, and make
sure that parameter is actually used.
Create the directory if it doesn't exist before copying files into it.
This fixes a problem in testNetconfig where
`IWD.copy_to_storage('ssidTKIP.psk', '/tmp/storage')`
would result in /tmp/storage being created as a file, rather than a
directory containing a file, and resulting in IWD failing to start with:
`Failed to create /tmp/storage`
runner.py creates /tmp/iwd but that doesn't account for IWD sessions
with a custom storage dir path.
a46707a595