Vultr: 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 Vultr provider

How Has This Been Tested?

Create the inlets exit node:

$ go run main.go create -p vultr --access-token "$VULTR_TOKEN"
Using provider: vultr
Requesting host: crazy-cray7 in LHR, from vultr
Host: 48732398, status: 
[1/500] Host: 48732398, status: none
[2/500] Host: 48732398, status: none
...
[204/500] Host: 48732398, status: active
inlets PRO TCP (0.8.3) server summary:
  IP: 136.244.64.63
  Auth-token: vQGanZLGBjleE4IacZc1TGDkL2c9JPImBotrclYl01DxC8ssKq2vLIRkwa1o8lVc
...

Connect client:

$ inlets-pro tcp client --url "wss://136.244.64.63:8123" \
>   --token "vQGanZLGBjleE4IacZc1TGDkL2c9JPImBotrclYl01DxC8ssKq2vLIRkwa1o8lVc" \
>   --upstream $UPSTREAM \
>   --ports $PORTS
2021/07/06 19:57:35 Starting TCP client. Version 0.8.3 - 205c311fde775723cf68b8116dacd7f428d243f8
2021/07/06 19:57:35 Licensed to: Johan Siebens <redacted>, expires: 85 day(s)
2021/07/06 19:57:35 Upstream server: localhost, for ports: 8000
inlets-pro client. Copyright Alex Ellis, OpenFaaS Ltd 2020
INFO[2021/07/06 19:57:35] Connecting to proxy                           url="wss://136.244.64.63:8123/connect"

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.

[alexellis]

That const should be lowercase, I've just realised that I wrote that in error, it wouldn't need a capital letter (package scope)