Linode: update Ubuntu version due to deprecation of 16.04

[jsiebens]

Signed-off-by: Johan Siebens johan.siebens@gmail.com

Description

• Update Ubuntu images to 18.04 for the Linode provider
• Set default region for Linode

How Has This Been Tested?

Create the inlets exit node:

go run main.go create -p linode --access-token $LINODE_TOKEN
Using provider: linode
Requesting host: cranky-swanson6 in eu-west, from linode
Host: 28305085, status: provisioning
[1/500] Host: 28305085, status: provisioning
...
[27/500] Host: 28305085, status: active
inlets PRO TCP (0.8.3) server summary:
  IP: 178.79.130.30
  Auth-token: lvsSgN9Fe3NXfJyu2KkwF17TXdgcjJWYseZzbHceUQz85ZL7A2dUGEhhyW6pQdnx

Connect client:

$ inlets-pro tcp client --url "wss://178.79.130.30:8123" \
>   --token "lvsSgN9Fe3NXfJyu2KkwF17TXdgcjJWYseZzbHceUQz85ZL7A2dUGEhhyW6pQdnx" \
>   --upstream $UPSTREAM \
>   --ports $PORTS
2021/07/06 20:16:47 Starting TCP client. Version 0.8.3 - 205c311fde775723cf68b8116dacd7f428d243f8
2021/07/06 20:16:47 Licensed to: Johan Siebens <redacted>, expires: 85 day(s)
2021/07/06 20:16:47 Upstream server: localhost, for ports: 8000

How are existing users impacted? What migration steps/scripts do we need?

no impact

Checklist:

I have:

• [x] updated the documentation and/or roadmap (if required)
• [x] read the CONTRIBUTION guide
• [x] signed-off my commits with git commit -s
• [x] added unit tests No unit tests exist for most other providers and there's not much logic.