component/defaults: use python3 by default

Change python2 softwares to use python2 explicitly and python3
softwares to use the default, but leave a few softwares (html5as,
html5as-base, fluent-bit, packer, seleniumrunner) using the default
python, as they were using python2 not because they only support python2
but because they don't have strong dependency to the python version
being used.