Required for handling the cases where set_options
is called on non-existing logical router port.
The option is added to parent class BaseSetOptionsCommand, so
if other apis relying on that need to expose if_exists then those
can be extended in future.
Partial-Bug: #2065701
Related-Bug: #2060163
Change-Id: I56685478214aae7b6d3a2a3187297ad4eb1869a3