don't use Etc.getlogin

I discovered that `mco` would become confused when called after a `su` or via Jenkins.

additionally, RDoc states: ["Returns the short user name of the currently logged in user. Unfortunately, it is often rather easy to fool getlogin(). Avoid getlogin() for security-related purposes."](http://ruby-doc.org/stdlib-1.9.3/libdoc/etc/rdoc/Etc.html#method-c-getlogin)

replacement code taken from https://github.com/embarkmobile/foreman_god/issues/1
This is a bit heavy.  What about this instead:

```
passwd = mock('passwd')
passwd.stubs(:name).returns('rspec')
Etc.stubs(:getpwuid).returns(passwd)
```