netplan generate incorrectly quotes WPA PSK hex

[Anurag Dulapalli]

This was discovered on an Ubuntu Core 18 running arm64 machine.

Please note that `wpa_supplicant` accepts (optionally) quoted plaintext strings as well as hex strings as field values for `psk=`. However, hex strings specifically are invalid unless they are un-quoted

Suppose the target `wpa_supplicant` compatible configuration is as follows:
```
network={
  ssid="<SSID_NAME>"
  psk=<PSK_HEX_STRING_UNQUOTED>
}
```

Then providing a `netplan` YAML configuration as follows, should result in the above:
```
network:
  ethernets:
    eth0:
      addresses: []
      dhcp4: true
  version: 2
  wifis:
    wlan0:
      access-points:
        "<SSID_NAME>":
          auth:
            key-management: psk
            password: <PSK_HEX_STRING_UNQUOTED>
      addresses: []
      dhcp4: true
      nameservers: {}

```

However, the actual resulting `wpa_supplicant` configuration by `netplan generate` is the following:
```
ctrl_interface=/run/wpa_supplicant
network={
  ssid="<SSID_NAME>"
  key_mgmt=WPA-PSK
  psk="<PSK_HEX_STRING_UNQUOTED>"
}
```

[Anurag Dulapalli (anurag-k4)]

There is a Pull Request available that addresses this issue, here:
https://github.com/CanonicalLtd/netplan/pull/118

[Łukasz Zemczak (sil2100)]

Thanks! As per PR comment, I have submitted a new PR for the other approach I mentioned:
https://github.com/CanonicalLtd/netplan/pull/120
What do you think?

[Lukas Märdian (slyon)]

Released as of netplan.io 0.99