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.

*I'm not sure if this is generally useful, but I needed to install a version of cube with a change not in the upstream version so I wanted to use:*
```
class { 'cube':
  cube_source => 'https://github.com/cwarden/cube/archive/collector-cors-support.tar.gz'
}
```