wordpress: Replace aufs with overlayfs

Note: This role does not clean up the old aufs directories. I could implement it, but I don’t think it’s worth it for a one-time task.

Things to do:

• unmount old
• remove fstab entries
• make sure *-files/.wh* are empty and remove them
• eventually remove aufs packages

approved this merge request

mentioned in commit 53f0fb50

merged