Merge lp://qastaging/~hopem/charm-helpers/lp1257491 into lp://qastaging/charm-helpers

Hi Ed

Please update the associated test as well;

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FAIL: It calls sgdisk correctly to zap disk
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Traceback (most recent call last):
  File "/home/jamespage/src/charms/charm-helpers/charm-helpers/tests/contrib/storage/test_linux_storage_utils.py", line 15, in test_zap_disk
    check_call.assert_called_with(['sgdisk', '--zap-all', '/dev/foo'])
  File "/usr/lib/python2.7/dist-packages/mock.py", line 835, in assert_called_with
    raise AssertionError(msg)
AssertionError: Expected call: check_call(['sgdisk', '--zap-all', '/dev/foo'])
Actual call: check_call(['sgdisk', '--zap-all', '--mbrtogpt', '/dev/foo'])

I was also pondering whether target format should be an option (MBR or GPT) but I guess this covers 100% of use cases right now.

Ta