Streamline release generation process

[erszcz]

Right now there's a release generated for each predefined configuration set. That's unnecessary and unwanted because of the time needed to generate a release.

However, reltool includes a templating support for files going into the release which is used for generating corresponding ejabberd.cfg files. That's convenient.

Solution:

• generate the release once to cut down build time,
• use mustache (or something similar) to generate customised ejabberd.cfg variants.