Support multiple python module instances

Support multiple python module instances

This commit adds proper support for multiple instances of the python
module: When more than one instance is added to the module list, the
first instance loads the first script specified in the `python:`
configuration section. The second instance loads the second script,
and so on.

When there are more module instances in the module list than there are
scripts in the `python:` section, an error is raised during
initialization and unbound won't start. When more scripts than module
instances are provided, the surplus scripts are ignored.