Implement changes entry generation from SCM

Introduces three new service XML parameters:

  changesgenerate - Flag to trigger, defaults to "disable"
  changesauthor   - Optional author of the changes entries, defaults to first email= entry in ~/.oscrc or "opensuse-packaging@opensuse.org"

All of these are optional and tar_scm will pick sane defaults or fail. Changes entry generation is disabled by default but can be opted in by adding <param name="changesgenerate">enable</param> to any _service file.

SCM agnostic but currently implemented only for the git SCM. Supports multiple tar_scm services in one _service file. Uses lxml if available to not destroy comments in the _service file (as the stdlib xml does).

The file _servicedata can service as a local datastore for source services. It's current layout is service-agnostic and only used by tar_scm.