Add common dir for shared agent code, add OVS lib.

Add common dir for shared agent code, add OVS lib.

bp quantum-agent-common

Adds a common directory that can be used for code shared by agents for
different plugins.  Also seeds this directory with an OVS library,
removing that code from the openvswitch plugin itself.  This code
can then be leveraged by other plugins (e.g., Ryu) who have similar code.

Also add a suite of mox-based tests for OVS lib.

Also add more powerful OVS flow expression builder as suggested by
salv-orlando, plus additional flow expression testing.

Note: the expectation is that this directory will be used for much of the
agent functionality that is similar to what Nova's nova/network/linux_net.py
file included, such as iptables manipulation, dhcp manipulation, etc.

People should be careful about changing code in this directory in a
non-backward compatible way, as other plugins may be using the code as well.

Change-Id: I8fd15ec6b8016e85a3f02e0d756a3fd61b1cab15