This patch introduces the needed fields for OpenvSwitch parameters
into ndparams and also provides the default values.

The parameters are:
ND_OVS: boolean, to show whether OpenvSwitch is enabled or not
ND_OVS_NAME: the name of the OpenvSwitch to create
ND_OVS_LINK: the physical link to the OpenvSwitch

While ND_OVS and ND_OVS_NAME are mandatory and default to FALSE and
constants.DEFAULT_OVS respectively, ND_OVS_LINK is optional and can
be left empty.
Adding a new node with OpenvSwitch and no physical interface might
or might not be what the user wants, so this case will result in a
warning to inform the user (see later patch in this series).

This patch also fixes unittests which are using these parameters.

Signed-off-by: Sebastian Gebhard <sege@fs.ei.tum.de>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>