Avoid trimming everything if the suffix length is zero

[georgebuckerfield]

I think this should fix #44

At the moment, if iam_linux_user_suffix is not set, the code ends up effectively trimming the username like so:

user.name[:-0]

This removes the entire username. Fixed by changing the trim to:

user.name[:len(user.name)-len(iam_linux_user_suffix)

This should just remove the iam_linux_user_suffix. I've made this change manually on a couple of servers and it seems to have resolved the issue and users are now being added to groups.

[kislyuk]

Thank you for finding and fixing this. Clearly I'm not very good at testing things :)