model-config image-stream not applying to model machine instances

I've just noticed that I'm getting 'released' images for my model instances, despite setting image-stream to daily. The controller instances, however, do use a daily image as expected. This is Juju 2.1.2 + OpenStack Provider against an Ocata undercloud.

#### On a user's model, the instance is based on the *release* image, when the *daily* image was expected.

jenkins@juju-10f68a-osci-13:~$ openstack server show f487418d-0fe8-46ec-a8e1-92ff859c9051 | grep image
| image | ubuntu-released/ubuntu-xenial-16.04-amd64-server-20170330-disk1.img (8b4dc7a6-11ad-44a0-86ca-9844d9763704) |

jenkins@juju-10f68a-osci-13:~$ openstack server show 5d799d64-cb31-475a-ac78-59fff602e0f3 | grep image
| image | ubuntu-released/ubuntu-xenial-16.04-amd64-server-20170330-disk1.img (8b4dc7a6-11ad-44a0-86ca-9844d9763704)

jenkins@juju-10f68a-osci-13:~$ juju switch auto-osci-sv08
auto-osci-sv08:admin/controller (no change)

jenkins@juju-10f68a-osci-13:~$ juju model-config
Attribute From Value
agent-metadata-url default ""
agent-stream controller proposed
agent-version model 2.1.2
apt-ftp-proxy default ""
apt-http-proxy default ""
apt-https-proxy default ""
apt-mirror default ""
automatically-retry-hooks controller false
default-series controller xenial
development default false
disable-network-management default false
enable-os-refresh-update controller false
enable-os-upgrade controller false
external-network default ""
firewall-mode default instance
ftp-proxy default ""
http-proxy default ""
https-proxy default ""
ignore-machine-addresses default false
image-metadata-url default ""
image-stream controller daily
logforward-enabled default false
logging-config model <root>=WARNING;unit=DEBUG
net-bond-reconfigure-delay default 17
network model 86125222-dd54-4df2-9421-235456fab193
no-proxy default ""
provisioner-harvest-mode default destroyed
proxy-ssh default false
resource-tags model {}
ssl-hostname-verification default true
storage-default-block-source model cinder
test-mode controller true
transmit-vendor-metrics controller false
use-default-secgroup controller true
use-floating-ip default false

.

#### On the controller model, the instance is based on the *daily* image as expected.

jenkins@juju-10f68a-osci-13:~$ juju switch controller
auto-osci-sv08:admin/auto-osci-sv08 -> auto-osci-sv08:admin/controller

jenkins@juju-10f68a-osci-13:~$ juju machines
Machine State DNS Inst id Series AZ
0 started 172.17.108.3 e872d229-1f50-4d2a-9263-b8ade337ea7e xenial nova

jenkins@juju-10f68a-osci-13:~$ openstack server show e872d229-1f50-4d2a-9263-b8ade337ea7e | grep image
| image | ubuntu-daily/ubuntu-xenial-daily-amd64-server-20170331-disk1.img (f9e9cdd6-be96-4996-9cbe-d458a645e906) |

jenkins@juju-10f68a-osci-13:~$ juju switch controller
auto-osci-sv08:admin/controller (no change)

jenkins@juju-10f68a-osci-13:~$ juju model-config
Attribute From Value
agent-metadata-url default ""
agent-stream controller proposed
agent-version model 2.1.2
apt-ftp-proxy default ""
apt-http-proxy default ""
apt-https-proxy default ""
apt-mirror default ""
automatically-retry-hooks controller false
default-series controller xenial
development default false
disable-network-management default false
enable-os-refresh-update controller false
enable-os-upgrade controller false
external-network default ""
firewall-mode default instance
ftp-proxy default ""
http-proxy default ""
https-proxy default ""
ignore-machine-addresses default false
image-metadata-url default ""
image-stream controller daily
logforward-enabled default false
logging-config model <root>=WARNING;unit=DEBUG
net-bond-reconfigure-delay default 17
network model 86125222-dd54-4df2-9421-235456fab193
no-proxy default ""
provisioner-harvest-mode default destroyed
proxy-ssh default false
resource-tags model {}
ssl-hostname-verification default true
storage-default-block-source model cinder
test-mode controller true
transmit-vendor-metrics controller false
use-default-secgroup controller true
use-floating-ip default false

.

#### Other info

jenkins@juju-10f68a-osci-13:~$ juju controllers
Use --refresh flag with this command to see the latest information.

Controller Model User Access Cloud/Region Models Machines HA Version
auto-osci-sv08* auto-osci-sv08 admin superuser serverstack/serverstack 2 1 none 2.1.2

jenkins@juju-10f68a-osci-13:~$ juju models
Controller: auto-osci-sv08

Model Cloud/Region Status Machines Cores Access Last connection
auto-osci-sv08* serverstack/serverstack available 24 48 admin 3 seconds ago
controller serverstack/serverstack available 1 4 admin just now
default serverstack/serverstack available 0 - admin 14 minutes ago