packaging: Make release number based on date

This patch changes the default release number of the packages generated
with "make rpm" so that they have the following format:

  0.1.$(shell date +%Y%m%d%H%M%S)

This has the advantage that builds in development environment can be
easily updated and also means that any release we do will always be
greater.

Change-Id: Ia9820caa71723e929cae5fb873013e52acce221c
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>