Add the host_name

[soif]

Add the host_name value so that the graph are displayed as a separate node.
Then all graphs are splitted in different categories.

The defaut node name is 'hg612', but can be overridden by setting the env.host_name in the /etc/munin/plugin-conf.d configuration file, ie:

 [hg612_*]
env.statsfile /var/lib/munin/huawei-hg612-munin-output.txt
env.host_name modem-hg612.local

enjoy

BTW some of the commits have been initiated by @nikdoof 🍻