Fix the OVS version check to work with upstream master versions of OVS.

This patch adds two functions to check version strings in the toplevel
functions file. The openvswitch_agent then uses these to compare versions
when checking for tunneling support. The tunneling version check now
also takes into account upstream master versions of Open vSwitch, which
the previous version check always failed on.

Fixes bug #1190734

Change-Id: I0102fb57f8ce5529169025efa21a0996ad68bef1
Merge "Fix the OVS version check to work with upstream master versions of OVS."