openvswitch: add option for failure mode

openvswitch: add option for failure mode

When Open vSwitch is configured to use a controller, but is unable to
connect to it, Open vSwitch will setup flows to allow all traffic, if
the failure mode is not configured, or set to standalone.

As this might be a security hazard, it is also possible to configure
Open vSwitch in a secure failure mode. Enabling this mode causes Open
vSwitch to drop all traffic if it is unable to connect to the
controller.

Redirect stderr of the command to /dev/null as it does not support the
--if-exists option.

Signed-off-by: Stijn Tintel <redacted>