extra_plugins config not written when changing install_method

[Laurent Sesquès]

When changing install_method (from deb to snap or the opposite), the 'extra_plugins.configured' doesn't get cleared, so the config for extra plugins doesn't get written to the new location (either /etc/telegraf/telegraf.d or /var/snap/telegraf/current/telegraf.d).
Clearing that flag when install_method is changed will fix it.