Add netaddr requirements for running ipv4|ipv6 filters

Running the ipv4 and ipv6 filters [1] requires the netaddr python
library to be installed on the Ansible *control node*, which ends up
being the executor.
These filters are very useful to determine if an IP is ipv4 or ipv6.

[1]: http://docs.ansible.com/ansible/latest/playbooks_filters_ipaddr.html

Change-Id: I800c7512fc60f9a302fb77cb061610430fcf8e49

requirements.txt

@@ -17,6 +17,7 @@ apscheduler>=3.0
 PrettyTable>=0.6,<0.8
 babel>=1.0
 ansible>=2.3.0.0,<2.4
+netaddr
 kazoo
 sqlalchemy
 alembic