debian: force-reload-kmod while package upgrading.

Currently, when we upgrade openvswitch packages, we do a restart
of userspace daemons automatically. This does not replace the
kernel module.

But almost everytime, we want to use the new kernel module
that comes with the new version. This means that we need to
manually do a "force-reload-kmod". This step, reloads the
kernel module and also restarts the userspace daemons. This gives
us a total of two restarts of userspace daemons. This is quite
expensive in a hypervisor with hundreds of VMs sending real traffic.
This also hurts the controller as it gets two reconnections in a short
amount of time.

With this patch, during a package upgrade, if the kernel module
on disk is different than the one that is loaded, we will
automatically do a force-reload-kmod while openvswitch-switch
is installed. If not, we will just do a "restart" like before.

One can install the kernel package first and then install the userspace
packages in 2 separate steps to enforce a single 'force-reload-kmod'.

If anyone wants to just restart the userspace package instead of
force-reload-kmod, they can set the value of OVS_FORCE_RELOAD_KMOD=no
while installing the package.
Ex: OVS_FORCE_RELOAD_KMOD=no dpkg -i openvswitch-switch*

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>