ImportError: No module named six when deploying on Precise

[Ryan Beisner]

When using this charm to deploy OpenStack (precise-icehouse, in conjunction with stable or next openstack charm sets):

# from mongodb unit log:
2014-12-10 16:37:28 DEBUG juju.worker.uniter modes.go:398 ModeInstalling local:precise/mongodb-30 exiting
2014-12-10 16:37:28 INFO juju.worker.uniter modes.go:396 ModeContinue starting
2014-12-10 16:37:28 INFO juju.worker.uniter modes.go:65 found queued "install" hook
2014-12-10 16:37:28 INFO juju.worker.uniter uniter.go:480 running "install" hook
2014-12-10 16:37:28 DEBUG juju.worker.uniter.filter filter.go:317 got unit change
2014-12-10 16:37:28 INFO install Traceback (most recent call last):
2014-12-10 16:37:28 INFO install File "/var/lib/juju/agents/unit-mongodb-0/charm/hooks/install", line 25, in <module>
2014-12-10 16:37:28 INFO install from charmhelpers.fetch import (
2014-12-10 16:37:28 INFO install File "/var/lib/juju/agents/unit-mongodb-0/charm/hooks/charmhelpers/fetch/__init__.py", line 5, in <module>
2014-12-10 16:37:28 INFO install from charmhelpers.core.host import (
2014-12-10 16:37:28 INFO install File "/var/lib/juju/agents/unit-mongodb-0/charm/hooks/charmhelpers/core/host.py", line 19, in <module>
2014-12-10 16:37:28 INFO install import six
2014-12-10 16:37:28 INFO install ImportError: No module named six
2014-12-10 16:37:28 ERROR juju.worker.uniter uniter.go:486 hook failed: exit status 1

...

# from deployer output:
2014-12-10 16:37:28 [DEBUG] deployer.env: Delta unit: mongodb/0 change:pending
2014-12-10 16:37:33 [DEBUG] deployer.env: Delta unit: mongodb/0 change:error
2014-12-10 16:37:33 [ERROR] deployer.env: The following units had errors:
   unit: mongodb/0: machine: 9 agent-state: error details: hook failed: "install"
2014-12-10 16:37:33 [INFO] deployer.cli: Deployment stopped. run time: 548.31

...

See also, similar mysql charm bug: https://bugs.launchpad.net/charms/+source/mysql/+bug/1398023