VPP Python language binding - plugin support

- Moved Python generator tool to tools directory
- Added build-vpp-api Makefile target
- Generator now only creates a Python representation of the .api
  the rest of the framework is in the vpp_papi script
- Each plugin has its own namespace.
- Plugin Python files are installed in vpp_papi_plugins for easy
  use inside the build tree.

Change-Id: I272c83bb7e5d5e416bdbd8a790a3cc35c5a04e38
Signed-off-by: Ole Troan <ot@cisco.com>

vpp-api/python/vpp_papi/__init__.py

@@ -1,2 +1,4 @@
 __import__('pkg_resources').declare_namespace(__name__)
-from .vpp_papi import *
+from . vpp_papi import *
+
+