import and install munin plugins from debian-ci-config

This copies the munin plugins from debian-ci-config as of 24458ea90331 ("Merge branch 'munin-no-template' into 'master'"), which was the moment where none of them were templates anymore.

The plugins are installed into the standard location /usr/share/munin/plugins. Since debian-ci-config installs plugins directly into /etc/munin/plugins, this will not affect the main installation.

The plugin-conf.d files are directly installed to the standard location /etc/munin/plugin-conf.d. These have the same content as in debian-ci-config for now.

Once this is deployed, debian-ci-config should stop installing the plugin-conf.d files and enable the plugins by placing symbolic links to the files in /usr/share/munin/plugins. Thus debian-ci-config can drop the actual plugins finishing this transition.