ifupdown: fixes to make addon scripts work via special config options

ifupdown: fixes to make addon scripts work via special config options

ifupdown2 can invoke scripts under /etc/network/if-*.d/*
in the required order with the required environment variables.
This patch includes fixes to execute these scripts.

The following attributes in /etc/network/ifupdown2/ifupdown.conf
can influence the execution behaviour of python-addon modules
and /etc/network/if-*.d/*

<ifupdown.conf>
addon_syntax_check=0

addon_scripts_support=1

addon_python_modules_support=1
</ifupdown.conf>

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>