Add xenial/bionic compatibility:
- Port hooks.py to Python 3
- Install Python 2.7 explicitly for turku-storage programs themselves
- Drop direct support for attic (dropped from Ubuntu, buggy), switch to link-dest on new deploys
- Add support for volumes=dir:/path/to/storage/dir (not technically needed for compatibility, but makes testing without volumes possible)

Reviewed-on: https://code.launchpad.net/~fo0bar/canonical-is-charms/turku-storage/+merge/344983
Reviewed-by: Tom Haddon <tom.haddon@canonical.com>