Fix wrong sysctl variable name

The bridge parameters should be net.bridge.* according to install
documentation at docs.openstack.org.

Change-Id: Ib29dd462e01f26e3fec48f0730ed322f2704cd8a
Closes-Bug: #1503416

ansible/roles/nova/tasks/config.yml

 ---
 - name: Disabling netfilter for bridges
-  sysctl: name="net.ipv4.bridge-nf-call-{{ item }}" value=1 sysctl_set=yes
+  sysctl: name="net.bridge-nf-call-{{ item }}" value=1 sysctl_set=yes
   with_items:
     - "iptables"
     - "ip6tables"