feat(python-plugin): prepend instead of append ModulePath to sys.path

When importing a module, you expect the plugin repertory to have higher priority
than your site-packages. It will prevent hard to debug clashing module names problems.
At least, if there is a problem, it will fail more loudly than "module did not register
any config callback"
doc(collectd-python): update
Merge pull request #751 from ptbrowne/prepend-sys-path

feat(python-plugin): prepend instead of append ModulePath to sys.path