Clean up bootstrap tests
Append --omnibus-version to --omnibus-options

Also add unit test to verify that the specified options will
be passed to the Omnibus installer.
Fix copy&paste error in PR #169

Sigh.
Remove singleton instance variable for #operating_system

The method is only called once.
Merge pull request #173 from tmatilai/omnibus_options

Append --omnibus-version to --omnibus-options