Replace dashes with underscores in python modules

It's not possible to import python modules with dashes in the name.
See http://bit.ly/2t7fcy for more information.

diff --git a/contrib/collectd-network.py b/contrib/collectd_network.py
similarity index 100%
rename from contrib/collectd-network.py
rename to contrib/collectd_network.py

diff --git a/contrib/collectd-unixsock.py b/contrib/collectd_unixsock.py
similarity index 100%
rename from contrib/collectd-unixsock.py
rename to contrib/collectd_unixsock.py