(#58) fix user impersonation in rspec-system tests

On Centos 6.4 `sudo -u USERNAME` is enough to impersonate a user
and use their mcollective tokens, but on ubuntu 12.10 `sudo -i -u
USERNAME` is required.  This is compatible with centos, so do it
that way everywhere.
Merge pull request #60 from richardc/bug/master/58

(#58) fix user impersonation in rspec-system tests