[VVP] Adding preload generation functionality

preload.py discovers and loads implementations of
AbstractPreloadGenerator from any module on sys.path prefixed with
preload_*

Initial support is provided for VNF-API and GR-API.  The templates
will provide a guide for users to provide their values.

Known limitations:
- No support for Contrail.  Preload will be created, but contrail
  parameters will be skipped. This will be addressed in the future.

Issue-ID: VVP-227
Signed-off-by: stark, steven <steven.stark@att.com>
Change-Id: I081d50ac379062fbf1bffebd687e920220d32571
Signed-off-by: Lovett, Trevor <trevor.lovett@att.com>
Signed-off-by: Lovett, Trevor (tl2972) <tl2972@att.com>