install fails due to None being passed to apt-key

OpenStack deployment testing started failing with mongodb charm revno 60 (MP 244156). I rolled back to 59 and confirmed successful U-J deployments with the stable and next charm sets.

All supported deployment test targets fail with same/similar error:
  utopic-juno
  trusty-juno
  trusty-icehouse
  precise-icehouse

Bear in mind that uosci is currently testing in a new/temporary environment while our lab is rebuilt, I am looking for someone to reproduce and confirm elsewhere.

## steps to reproduce failure
Using this bundle: http://bazaar.launchpad.net/~ost-maintainers/openstack-charm-testing/trunk/view/head:/default.yaml
  juju-deployer -B -c default.yaml -d trusty-juno

## deployer
20:45:30 2014-12-09 20:45:30 [DEBUG] deployer.env: Delta unit: mongodb/0 change:error
20:45:30 2014-12-09 20:45:30 [DEBUG] deployer.env: Delta unit: openstack-dashboard/0 change:pending
20:45:30 2014-12-09 20:45:30 [DEBUG] deployer.env: Delta machine: 19 change:started
20:45:30 2014-12-09 20:45:30 [ERROR] deployer.env: The following units had errors:
20:45:30 unit: mongodb/0: machine: 9 agent-state: error details: hook failed: "install"
20:45:30 2014-12-09 20:45:30 [INFO] deployer.cli: Deployment stopped. run time: 815.81

## mongodb unit
2014-12-09 20:45:27 INFO install gpg: "None" not a key ID: skipping
2014-12-09 20:45:27 INFO install Traceback (most recent call last):
2014-12-09 20:45:27 INFO install File "/var/lib/juju/agents/unit-mongodb-0/charm/hooks/install", line 1367, in <module>
2014-12-09 20:45:27 INFO install hooks.execute(sys.argv)
2014-12-09 20:45:27 INFO install File "/var/lib/juju/agents/unit-mongodb-0/charm/hooks/charmhelpers/core/hookenv.py", line 479, in execute
2014-12-09 20:45:27 INFO install self._hooks[hook_name]()
2014-12-09 20:45:27 INFO install File "/var/lib/juju/agents/unit-mongodb-0/charm/hooks/install", line 730, in install_hook
2014-12-09 20:45:27 INFO install add_source(config('source'), config('key'))
2014-12-09 20:45:27 INFO install File "/var/lib/juju/agents/unit-mongodb-0/charm/hooks/charmhelpers/fetch/__init__.py", line 231, in add_source
2014-12-09 20:45:27 INFO install key])
2014-12-09 20:45:27 INFO install File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
2014-12-09 20:45:27 INFO install raise CalledProcessError(retcode, cmd)
2014-12-09 20:45:27 INFO install subprocess.CalledProcessError: Command '['apt-key', 'adv', '--keyserver', 'hkp://keyserver.ubuntu.com:80', '--recv', u'None']' returned non-zero exit status 2
2014-12-09 20:45:27 ERROR juju.worker.uniter uniter.go:486 hook failed: exit status 1