collectd: Support config of mqtt plugin

mqtt plugin is already built and shipped in
collectd-mod-mqtt, however it is not possible to configure it via
uci currently, instead having to rely on populating the config file manually.

Add support by adding 2 functions, process_mqtt() and
process_mqtt_block(). First one just enables/disables the plugin.
The second one, in the spirit of the curl plugin, adds support for
populating multiple <Publish> and <Subscribe> blocks under <Plugin mqtt>
with support for some parameters. Those are:

* blocktype. Publish or Subscribe. Mandatory
* name. The name of the block. Mandatory
* Host. Mandatory
* Port. Optional
* User. Optional
* Password. Optional
* ClientId. Optional
* QoS. Optional
* Prefix. Optional
* Retain. Optional
* StoreRates. Optional
* CleanSession. Optional
* Topic. Optional

Bump PKG_RELEASE per comments in PR

Signed-off-by: Alexandros Kosiaris <akosiaris@gmail.com>

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