Use list comprehension in create_bulk_networks

There is no need to pass size of list to create_bulk_networks in
network_client_base since it can be implemented with list comprehension.

Change-Id: I7eb8c7725d9569ad7c2fe853b72a51d10fe65bd1
Merge "Use list comprehension in create_bulk_networks"
Merge branch 'master' of https://github.com/openstack/tempest