collectd: add mqtt plugin support

Add explicit support for the mqtt (mosquitto) plugin.
Place it under the write category even though it can read as well, since
it will normally be used to write (publish) rather than read
(subscribe).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>