Merge the FiledVersionTagger into the base VersionTagger.

This allows the behaviour to be used in just about any tagger sub-class.

Behaviour will be triggered on the presence of the 'version_template'
section in tito.props.

Renamed config:
  version -> version_template (section)
  file -> destination_file

Separated the replacement of version and release, as caller may just
want one or the other. (it works fine if only one of the variables is
present in your template file) Release is more of an rpm concept so may
not always be wanted in this context.

Removed the notion of default template files, if you want to use this
you need to specify the configuration for it.

Added a functional test to make sure I keep it working.