Remove DisplayName option

Remove the option to set an environ user's display name from the
AddEnvirionmentUser function. Update call sites and expected test
results.
Merge branch 'master' of https://github.com/juju/juju into remove-display-name
Merge pull request #691 from waigani/remove-display-name

Remove DisplayName option

Remove the option to set an environ user's display name from the
AddEnvirionmentUser function. Update call sites and expected test
results.