Fix docker_container tests (#549)

* Stop using ansible.netcommon.next_nth_usable.

It requires netaddr to be installed before running the playbook, which isn't the case in CI.
This undoes https://github.com/ansible/ansible/pull/62928/commits/cf61474c3bb2f5d7de6fb0726590de4ddbe4b18b.

* Use own filter to avoid needing netaddr installed.

tests/integration/targets/docker_container/tasks/main.yml

@@ -9,11 +9,6 @@
 - debug:
     msg: "Using container name prefix {{ cname_prefix }}"
 
-# Install netaddr
-- name: Install netaddr for ipaddr filter
-  pip:
-    name: netaddr
-
 # Run the tests
 - block:
   - include_tasks: run-test.yml