Charm hooks fails due to netifaces newer version

[Gui Maluf Balzana]

While changing a config on graylog charm I got hit by

```
unit-graylog-0: 12:03:23 ERROR unit.graylog/0.juju-log Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-graylog-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-graylog-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-graylog-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-graylog-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-graylog-0/charm/reactive/graylog.py", line 238, in configure_graylog
    validate_api_uri("{}://0.0.0.0:{}/".format(get_api_protocol(), api_port)),
  File "lib/charms/layer/graylog/utils.py", line 63, in validate_api_uri
    netloc="{}:{}".format(get_ip_address(), parsed_url.port)
  File "lib/charms/layer/graylog/utils.py", line 81, in get_ip_address
    iface = netifaces.gateways()["default"][netifaces.AF_INET][1]
KeyError: 2
```

This showed up to be due to netifaces version in the virtual env of the charm

```
# netifaces==0.10.4
python3 -c 'import netifaces ; print(netifaces.gateways()["default"])'
{2: ('10.116.72.11', 'eth0'), 10: ('fa80::111:cdaf:eee8:19aa', 'eth1')}

# netifaces==0.11.0
/var/lib/juju/agents/unit-graylog-?/.venv/bin/python -c 'import netifaces ; print(netifaces.gateways()["default"])'
{10: ('fa80::111:cdaf:eee8:19aa', 'eth1')}
```

Downgrading netifaces<0.11 fixed the issue

```
/var/lib/juju/agents/unit-graylog-?/.venv/bin/python -m pip install 'netifaces<0.11'
```

[Mert Kirpici (mertkirpici)]

Root cause analysis shows that starting from netifaces v0.11.0:
1 - netifaces only considers routes as default if they are defined on the main table
2 - netifaces selects the route with the lowest metric(highest prio) to show as the "default"
3 - combining 1 and 2: if the default route with the lowest metric is not on the main table, netifaces.gateways()["default"] returns empty
4 - charm-advanced-routing introduces a new table called "apitable" upon installation with a default route on it
5 - the issue is observed when the default route with the lowest metric is not on the main table. Hence the error:

iface = netifaces.gateways()["default"][netifaces.AF_INET][1]
KeyError: 2

where 2 represents netifaces.AF_INET.

The issue can be resolved by any of the following:
- making sure the default route with the lowest metric is on the main table.
- pinning down the netifaces package
- proposing a patch to netifaces-plus if this _should_ not be the right behavior.(I am not exactly convinced that the default route MUST be in the main table..)

Relating the bug to the charm-advanced-routing project as well

[Andrea Ieri (aieri)]

Removing advanced-routing since this bug does not affect it (though it can be triggered by its presence)