tests: fix test_osds_listen_on_* tests

tests: fix test_osds_listen_on_* tests

the `test_osds_listen_on_*` consider OSDs will always listen on tcp port
with consecutive tcp port number starting from `6800`.

Eg.
If you have 2 OSDs, tests will assume it should listen on 2 ports for each
network (`public_network` and `cluster_network`), therefore:
`6800, 6801, 6802, 6803`

but sometime it doesn't happen this way and you can get OSDs listening
on tcp port like this :

`6800, 6801, 6802, 6805`

Then the test are failing while it shouldn't.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>