add collectd network plugin
add collectd network plugin

In order to see network metrics, we need to install the collectd plugin. Technically,
we could install this en lieu of collectd, as I am pretty sure it would pull collectd
as a dependency. But in my opinion, installing it after collectd makes it clearer what
is going on, even though it is slightly less efficient.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@scylladb.com>

cloudius-systems/cassandra-test-and-deploy,scylladb/cassandra-test-and-deploy,scylladb/cassandra-test-and-deploy,cloudius-systems/cassandra-test-and-deploy