change in source config does not change cloud-archive.list

[Xav Paice]

A cloud was upgraded from Trusty/Mitaka to Xenial/mitaka. The charm was configured with:

juju config ceph-mon --reset source

Then dist-upgraded from Trusty to Xenial.

We then switched source, using:

juju config ceph-mon source=cloud:xenial-newton

I expected the content of /etc/apt/sources.list.d/cloud-archive.list to contain the newton archive (even though there's no new packages for Ceph in it), however the content was the one left over from prior to the Xenial upgrade (commented out):

# Ubuntu Cloud Archive
# deb http://ubuntu-cloud.archive.canonical.com/ubuntu trusty-updates/mitaka main

[Xav Paice (xavpaice)]

Added Keystone, although the config item is named differently the same issue applies. Likely a charmhelpers issue.