Load br_netfilter module in nova-cell role

The nova-cell role sets the following sysctls on compute hosts, which
require the br_netfilter kernel module to be loaded:

    net.bridge.bridge-nf-call-iptables
    net.bridge.bridge-nf-call-ip6tables

If it is not loaded, then we see the following errors:

    Failed to reload sysctl:
    sysctl: cannot stat /proc/sys/net/bridge/bridge-nf-call-iptables: No such file or directory
    sysctl: cannot stat /proc/sys/net/bridge/bridge-nf-call-ip6tables: No such file or directory

Loading the br_netfilter module resolves this issue.

Typically we do not see this since installing Docker and configuring it
to manage iptables rules causes the br_netfilter module to be loaded.
There are good reasons [1] to disable Docker's iptables management
however, in which case we are likely to hit this issue.

This change loads the br_netfilter module in the nova-cell role for
compute hosts.

[1] https://bugs.launchpad.net/kolla-ansible/+bug/1849275



Co-Authored-By: Dincer Celik <hello@dincercelik.com>

Change-Id: Id52668ba8dab460ad4c33fad430fc8611e70825e