Make agent use installed OVS library

These changes allow agent to build against the OVS library
fork at github.com:noironetworks/ovs.git ("lib" branch):
- Makefile now uses pkgconfig to check for OVS.
- Changed the #includes to use the correct header locations.
- Added some defines to the build command to make CentOS builds
work.
- Minor change in ActionBuilder due to upstream changes in
ofp-actions.
- Fixed a bug in flow-executor (upstream OVS code
seems to have become stricter).

Change-Id: I9505781230f0dfa75173bd3c7f9a25116f3505a9
Signed-off-by: Amit Bose <bose@noironetworks.com>