unprivileged containers: use next available nic name if unspecified

Rather than always using eth0.  Otherwise unpriv containers cannot have
multiple lxc.network.type = veth's without manually setting
lxc.network.name =.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Support openvswitch bridges

We detect whether ovs-vsctl is available.  If so, then we support
adding network interfaces to openvswitch bridges with it.

Note that with this patch, veths do not appear to be removed from the
openvswitch bridge.  This seems a bug in openvswitch, as the veths
in fact do disappear from the system.  If lxc is required to remove
the port from the bridge manually, that becomes more complicated
for unprivileged containers, as it would require a setuid-root
wrapper to be called at shutdown.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>