Rename infrastructure_users, infrastructure_user

From I36114be2359f06e004de2911129c76d3c031fd3e Scott said that we should
never have more than one user. This re-works infrastructure_users to be
singular and its two callpoints to expect a str rather than a list.

BUG=chromium-os:26171
TEST=pylint + ran lab_deploy

Change-Id: I6d7a111aea9305c8bd774d6331aaa3c5c270106a
Reviewed-on: https://gerrit.chromium.org/gerrit/42268
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Scott Zawalski <scottz@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
4 files changed