collectd: Support configuration of write_http plugin

write_http plugin is already built and shipped in
collectd-mod-write_http, 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_write_http() and
process_write_http_node(). First one just enables/disables the plugin.
The second one, in the spirit of the curl plugin, adds support for
populating multiple <Node> elements under <Plugin write_http> with
support for a few parameters. Those are:

* name. The name of the <Node>. Mandatory
* URL. Mandatory
* Format. Optional.
* User. Optional.
* Password. Optional.
* Timeout. Optional.
* BufferSize. Optional.

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