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
Merge "Encrypt scrubber marker files"