Encrypt scrubber marker files

Files written to scrubber_datadir can contain credentials in plain text.

Update schedule_delayed_delete_from_backend to make use of
metadata_encryption_key to encrypt the uri before writing it out.

Fixes LP Bug #1112586

Change-Id: I03ff36f6b57f58a5e5de28bf8d48e7ce0216e5b3
Update HACKING.rst per recent changes

Added "is not" usage with examples
Fixed "not in" usage description
* https://review.openstack.org/#/c/20877/

Change-Id: I89fbc62a8b31bc7ef750cc1e80f73b9c1e366fd3
Merge "Update HACKING.rst per recent changes"
Merge "Encrypt scrubber marker files"