Support collect-logs from a non-/ root

This bug was originally filed in Launchpad as [LP: #1809139](https://bugs.launchpad.net/cloud-init/+bug/1809139)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2018-12-19T16:04:25.333356+00:00

date_fix_committed = None

date_fix_released = None

id = 1809139

importance = wishlist

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1809139

milestone = None

owner = oddbloke

owner_name = Dan Watkins

private = False

status = triaged

submitter = oddbloke

submitter_name = Dan Watkins

tags = []

duplicates = []

_Launchpad user **Dan Watkins(oddbloke)** wrote on 2018-12-19T16:04:25.333356+00:00_

When debugging instance launch failures, one trick we use is to mount the disk of the failed instance to another instance, and investigate the logs there. Most of the time, we can use chroot to run `cloud-init collect-logs` in the failed instance's root. However, if we are attempting to debug across architectures (most recently, debugging arm64 AWS boot failures either locally or on an amd64 AWS instance), then using chroot becomes decidedly non-trivial.

Could we add an option to specify the root in which collect-logs operates, to make collecting this data easier in these cases?