Allow cube package source to be overridden

By default, install default cube package from npm.  cube_source can be
used to override the default package with a URL to an alternative cube
tarball.
Update tests for cube_source parameter

If cube_source is not set, the latest cube package should be be
installed from npm.

If cube_source is set, only verify that the package is installed.
Merge pull request #1 from cwarden/source-override-parameter

Allow cube package source to be overridden