freebsd: introduce the freebsd renderer (#61)

* freebsd: introduce the freebsd renderer

Refactoring of the FreeBSD code base to provide a real network renderer
for FreeBSD.
Use the generic update_sysconfig_file() from rhel_util to handle the
access to /etc/rc.conf.
Interfaces are not automatically renamed by FreeBSD using
the following configuration in /etc/rc.conf:

```
ifconfig_fxp0_name="eth0"
```

* freesd: use regex named groups

Reduce the complexity of `get_interfaces_by_mac_on_freebsd()` with
named groups.

* freebsd: breaks up _write_network() in tree small functions

- `_write_ifconfig_entries()`
- `_write_route_entries()`
- `_write_resolve_conf()`

* extend find_fallback_nic() to support FreeBSD

this uses `route -n show default` to find the default interface

* freebsd: use dns keys from NetworkState class

The NetworkState class (settings instance) exposes the DNS configuration
in two keys:

- `dns_nameservers`
- `dns_searchdomains`

On OpenStack, these keys are set when a global DNS server is set. The
alternative is the `dns_nameservers` and `dns_search` keys from each
subdomain. We continue to read those.

* freebsd: properly target the /etc/resolv.conf file

* freebsd: ignore 'service routing restart' ret code

On FreeBSD 10, the restart of routing and dhclient is likely to fail because
- routing: it cannot remove the loopback route, but it will still set up
  the default route as expected.
- dhclient: it cannot stop the dhclient started by the netif service.
In both case, the situation is ok, and we can proceed.

* freebsd: handle case when metadata MAC local locally

Handle the case where the metadata configuration comes with a MAC that
does not exist locally.

See:
- https://github.com/canonical/cloud-init/pull/61/files/635ce14b3153934ba1041be48b7245062f21e960#r359600604
- https://github.com/canonical/cloud-init/pull/61/files/635ce14b3153934ba1041be48b7245062f21e960#r359600966

* freebsd: show up a warning if several subnet found

The FreeBSD provider currently only allow one subnet per interface.

* freebsd: honor the target parameter in _write_network

* freebsd: log when a bad route is found

* freebsd: pass _postcmds to start_services()

* freebsd: updatercconf() is depercated

Replace `updatercconf()` by `rhel_util.update_sysconfig_file()`.

* freebsd: ensure gateway is ipv4 before using it

With the legacy ENI format, an IPv6 gateway may be pushed. This instead
of the expected IPv4.

* freebsd: find_fallback_nic, support FB10

On FreeBSD <= 10, `ifconfig -l` ignores the down interfaces.

* freebsd: use util.target_path() to load resolv.conf

Ensure we access `/etc/resolv.conf`, not `etc/resolv.conf`.

* freebsd: skip subnet without netmask

Those are likely to be either invalid of in IPv6 format. IPv6 support
will be addressed later in a new patchset.

* freebsd: get_devicelist returns netif list

Ensure `get_devicelist()` returns the list of known netif on FreeBSD.

* replace rhel_util.update_sysconfig_file wrapper call, with a wrapper function

* reverse if condition to remove an indent

Co-authored-by: Igor Galić <me+github@igalic.co>

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