Deploying neutron-gateway fails if ipv6 is disabled on the host or instance

When deploying cs:openstack-base, if ipv6 is disabled on the target nodes at the kernel level, neutron-gateway will fail with the following message in the logs, and the hook will exit 1, instead of 0, causing the unit to go into 'error' state:

2020-06-05 18:38:13 DEBUG config-changed net.ipv4.neigh.default.gc_thresh1 = 128
2020-06-05 18:38:13 DEBUG config-changed net.ipv4.neigh.default.gc_thresh2 = 28672
2020-06-05 18:38:13 DEBUG config-changed net.ipv4.neigh.default.gc_thresh3 = 32768
2020-06-05 18:38:13 DEBUG config-changed sysctl: cannot stat /proc/sys/net/ipv6/neigh/default/gc_thresh1: No such file or directory
2020-06-05 18:38:13 DEBUG config-changed sysctl: cannot stat /proc/sys/net/ipv6/neigh/default/gc_thresh2: No such file or directory
2020-06-05 18:38:13 DEBUG config-changed sysctl: cannot stat /proc/sys/net/ipv6/neigh/default/gc_thresh3: No such file or directory
2020-06-05 18:38:13 DEBUG config-changed net.nf_conntrack_max = 1000000
2020-06-05 18:38:13 DEBUG config-changed net.netfilter.nf_conntrack_buckets = 204800
2020-06-05 18:38:13 DEBUG config-changed net.netfilter.nf_conntrack_max = 1000000
2020-06-05 18:38:13 DEBUG config-changed none
2020-06-05 18:38:13 DEBUG config-changed Traceback (most recent call last):
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/config-changed", line 387, in <module>
2020-06-05 18:38:13 DEBUG config-changed main()
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/config-changed", line 379, in main
2020-06-05 18:38:13 DEBUG config-changed hooks.execute(sys.argv)
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/charmhelpers/core/hookenv.py", line 943, in execute
2020-06-05 18:38:13 DEBUG config-changed self._hooks[hook_name]()
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 1706, in wrapped_f
2020-06-05 18:38:13 DEBUG config-changed stopstart, restart_functions)
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/charmhelpers/core/host.py", line 741, in restart_on_change_helper
2020-06-05 18:38:13 DEBUG config-changed r = lambda_f()
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 1705, in <lambda>
2020-06-05 18:38:13 DEBUG config-changed (lambda: f(*args, **kwargs)), __restart_map_cache['cache'],
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/charmhelpers/contrib/hardening/harden.py", line 93, in _harden_inner2
2020-06-05 18:38:13 DEBUG config-changed return f(*args, **kwargs)
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/config-changed", line 147, in config_changed
2020-06-05 18:38:13 DEBUG config-changed '/etc/sysctl.d/50-quantum-gateway.conf')
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/charmhelpers/core/sysctl.py", line 75, in create
2020-06-05 18:38:13 DEBUG config-changed raise e
2020-06-05 18:38:13 DEBUG config-changed File "/var/lib/juju/agents/unit-neutron-gateway-0/charm/hooks/charmhelpers/core/sysctl.py", line 69, in create
2020-06-05 18:38:13 DEBUG config-changed check_call(call)
2020-06-05 18:38:13 DEBUG config-changed File "/usr/lib/python3.6/subprocess.py", line 311, in check_call
2020-06-05 18:38:13 DEBUG config-changed raise CalledProcessError(retcode, cmd)
2020-06-05 18:38:13 DEBUG config-changed subprocess.CalledProcessError: Command '['sysctl', '-p', '/etc/sysctl.d/50-quantum-gateway.conf']' returned non-zero exit status 255.
2020-06-05 18:38:13 ERROR juju.worker.uniter.operation runhook.go:136 hook "config-changed" (via explicit, bespoke hook script) failed: exit status 1

The machine in this case, is maas-provisioned, and contains the kernel arg 'ipv6.disable=1', among others, which is inherited by the machines, VMs and units that maas and juju deploy.

Removing this arg from the neutron-gateway/0 node, re-running 'update-grub' and rebooting that node, allows the charm hooks to continue without error (after 'juju resolved neutron-gateway/0' of course).

Ideally, if ipv6 is disabled, the ipv6-specific tunables should be excluded from the hook, and only the ipv4 tunables applied.