get_user_by_name request does not work as expected

[plribeiro3000]

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It looks like Identity.get_user_by_name returns the full list of users currently and tests fail against a real installation.

I believe @drbrain noted this first (see b430ec6a20eb1216130df5024e32d18c4a60c3d6), so the question is, should we tweak the request to return the user requested or should we remove it?

@dprince?

I'd like to open a pull request if there's consensus around this feature.

Original issue opened by @rubiojr at fog/fog#1622.

[plribeiro3000]

cc/ @rubiojr @dprince @geemus