lacp: Option to toggle "Require successful LACP negotiation when configured"

Commit bdebeece5 (lacp: Require successful LACP negotiations when configured.)
makes successful LACP negotiation mandatory. This patch makes it configurable.

Several of the physical switches that support LACP block all traffic for ports
that are configured to use LACP, until LACP is negotiated with the host. When
configuring a LACP bond on a OVS host (eg: XenServer), this means that there
will be an interruption of the network connectivity between the time the ports
on the physical switch and the bond on the OVS host are configured. The
interruption may be relatively long, if different people are responsible for
managing the switches and the OVS host.

Such network connectivity failure can be avoided if LACP can be configured on
the OVS host before configuring the physical switch, and having the OVS host
fall back to a bond mode (active-backup) till the physical switch LACP
configuration is complete. An option "lacp-fallback-ab" is introduced with this
patch to provide such behavior on openvswitch.