Make merge.py installable as a package using pbr

Now that merge.py is invokable from another script
(Ia6b6416fe10358d23f2b120283eecaf4c1178cfd) and from comments at that
review, it makes sense to offer a nicer way to consume the merge
functionality.

Once you git clone tripleo-heat-templates you can python setup.py
install and get /usr/bin/tripleo_heat_merge as well as a
tripleo_heat_merge package in python2.7/site-packages.

Makefile edits required because we moved merge.py into the
tripleo_heat_merge directory for the packaging.

Change-Id: I587fa7a826f93f89e8e5c266af7f5765438fe738