[matter]: Force the use of a python 2 version

Default gentoo (maybe also sabayon?) installs nowaday (e)select python 3.3 as the global python implementation. This breaks matter on these systems, unless python-2.7 is (e)selected as the implementation.

Using /usr/bin/python2 instead, forces to use the (e)selected version 2 of python and makes it work with the new default of python-3.*.