add missing %(loc)s template for install_target = 'easy_install' in PythonPackage

[boegel]

@SethosII We overlooked this in #1341, without this the (crucial) . is missing at the end of the python setup.py easy_install . command, which leads to an error like:

== 2018-01-12 22:52:26,479 run.py:183 INFO running cmd:  /usr/bin/python setup.py easy_install --prefix=/user/home/gent/vsc400/vsc40023/eb_phanpyscratch/CO7/sandybridge/software    /vsc-base/2.5.1  --no-deps  --zip-ok
...
error: No urls, filenames, or requirements specified (see --help)