cloud-init clean should persist custom scripts subdirs

This bug was originally filed in Launchpad as [LP: #1788961](https://bugs.launchpad.net/cloud-init/+bug/1788961)

Launchpad details

affected_projects = []

assignee = chad.smith

assignee_name = Chad Smith

date_closed = None

date_created = 2018-08-24T23:00:36.494065+00:00

date_fix_committed = None

date_fix_released = None

id = 1788961

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1788961

milestone = None

owner = chad.smith

owner_name = Chad Smith

private = False

status = triaged

submitter = chad.smith

submitter_name = Chad Smith

tags = []

duplicates = []

_Launchpad user **Chad Smith(chad.smith)** wrote on 2018-08-24T23:00:36.494065+00:00_

Some images contain custom scripts that we need to preserve across 'cloud-init clean' calls.