Network check_prerequisite timeout

ospurge fails after 120 seconds because there are too many network elements to delete that the check_prerequisite() expires before gathering all of them:

$ ospurge --verbose --purge-project myproject --insecure

INFO:root:2020-06-02 07:19:31,875:Going to delete Port (id='c5afe26a-9ff8-48d3-9b8f-b826634d5d87', network_id='9de5b09f-f02f-4048-a0b8-f588f44214e7, device_owner='')'
INFO:root:2020-06-02 07:19:32,687:Waiting for check_prerequisite() in Networks
INFO:root:2020-06-02 07:19:32,994:Going to delete Port (id='c5e140d9-aa21-4e79-909a-48fc5613bc6a', network_id='cb3352dc-dea9-4be2-9f4c-754ecf33bd24, device_owner='')'
INFO:root:2020-06-02 07:19:34,331:Going to delete Port (id='c5ecbd86-f78e-4eec-a7b9-876bc326f2b8', network_id='bfed3f97-9fc0-4a88-b68e-2f7a2eb4afef, device_owner='')'
INFO:root:2020-06-02 07:19:35,657:Going to delete Port (id='c69807ce-64ec-4152-a02c-60d4d9dbb8bd', network_id='3a9d7aa7-b762-4909-97e4-1d3536739541, device_owner='')'
INFO:root:2020-06-02 07:19:36,725:Going to delete Port (id='c6b03b50-d17d-4728-8518-ac31c3aa2245', network_id='cc8ea2bd-28c9-4b59-8b1e-ecc462cdd1b2, device_owner='')'
INFO:root:2020-06-02 07:19:38,088:Going to delete Port (id='c70f964d-ad23-4569-a1cf-43e285a343b0', network_id='5075ecad-6237-4614-bffb-cbded9c477dd, device_owner='')'
INFO:root:2020-06-02 07:19:39,472:Going to delete Port (id='c7213c9d-ee18-48f1-9550-f35f45397ec3', network_id='7c020660-7370-415c-b9af-ef5c2bb5d637, device_owner='')'
INFO:root:2020-06-02 07:19:40,510:Going to delete Port (id='c7401d64-358d-407c-9266-9d51592fad91', network_id='8557daa6-c32d-4fd5-ac0c-8a6588fd6c5b, device_owner='')'
ERROR:root:2020-06-02 07:19:40,695:Can't deal with Networks: TimeoutError('Timeout exceeded waiting for check_prerequisite()',)

timeout is hardcoded to 120 seconds:
https://opendev.org/x/ospurge/src/branch/master/ospurge/resources/base.py#L152-L153

it would be required to be able to set it by a CLI arg.