Change-Id: Id4b0b405872457bd8b20f450e4031d6808d3cf59

Change-Id: Ic39fa0f18e0d9aeca0ef73ae6d985d12d15a1c3a

This is to comply to a comment from @cznewt in
https://github.com/tcpcloud/salt-formula-heka/pull/24.

This patch replaces the "interface" collectd plugin by the "netlink" one. The
"netlink" plugin provides the same metrics as "interface" but plus other
metrics such as the number of dropped packets.