Replaced code in routers_info module with openstacksdk function

Replaced custom code for interface listing with call to openstacksdk.
The original idea was to reduce the number of calls to the OpenStack
API but this kind of optimization is better to be implemented in the
SDK itself [1]. Reimplementing code like this increases our
maintenance burden, does not help other SDK users and increases the
likeliness of bugs. For example, variable allowed_device_owners
introduced a bug, it is not 'network_router_interface_distributed'
but 'network:router_interface_distributed'.

[1] https://review.opendev.org/c/openstack/openstacksdk/+/849967

Change-Id: I9c52de03c53ef29d7cecdf26253c0c00a7cf3689