revise RPM version approach

This approach abandons using the output of "git describe" to produce a version string in favor of either and explicit VERSION and RELEASE variable in setup.py or, if RELEASE is "0", a timestamp.  This mirrors a change we made in image factory, which is where the earlier RPM approach came from.

This avoids some odd issues with RPM version math that arose with the earlier approach, particularly when squashing commits before merging them into master.