Same thing happened again at a customer, this time with nova-cloud-controller:
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 The following packages were automatically installed and are no longer required:
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40   libfreetype6 os-prober
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 Use 'apt-get autoremove' to remove them.
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 The following NEW packages will be installed:
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40   python-netifaces
unit-mongodb-0[1005]: 2015-09-10 22:09:21 INFO unit.mongodb/0.install logger.go:40 Selecting previously unselected package python-bson.
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 Need to get 11.3 kB of archives.
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 After this operation, 63.5 kB of additional disk space will be used.
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 WARNING: The following packages cannot be authenticated!
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40   python-netifaces
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 E: There are problems and -y was used without --force-yes
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 Traceback (most recent call last):
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/install", line 45, in <module>
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40     from charmhelpers.contrib.openstack.utils import (
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 31, in <module>
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40     from charmhelpers.contrib.network import ip
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/charmhelpers/contrib/network/ip.py", line 36, in <module>
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40     import netifaces
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 INFO unit.nova-cloud-controller/0.install logger.go:40 ImportError: No module named netifaces
unit-nova-cloud-controller-0[1012]: 2015-09-10 22:09:21 ERROR juju.worker.uniter.operation runhook.go:103 hook "install" failed: exit status 1


Calling apt-get install? If you don't call apt-get update soon before, you will fail.