selftests: forwarding: add ability to assemble NETIFS array by driver name

Allow driver tests to work without specifying the netdevice names.
Introduce a possibility to search for available netdevices according to
set driver name. Allow test to specify the name by setting
NETIF_FIND_DRIVER variable.

Note that user overrides this either by passing netdevice names on the
command line or by declaring NETIFS array in custom forwarding.config
configuration file.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>