add extra flag to override default plan or size of the exit-node

[jsiebens]

Description

This PR adds an additional flag to override the default plan or size of the exit node

How Has This Been Tested?

Successfully created an exit node with a different plan, e.g. Scaleway Stardust:

$ go run main.go create -p scaleway --region=nl-ams-1 --plan=STARDUST1-S ...
Using provider: scaleway
Requesting host: strange-wiles1 in nl-ams-1, from scaleway
Host: d1622777-0c3e-4a03-975c-6825e42d9163, status: stopped
[1/500] Host: d1622777-0c3e-4a03-975c-6825e42d9163, status: starting
[2/500] Host: d1622777-0c3e-4a03-975c-6825e42d9163, status: starting
...
[18/500] Host: d1622777-0c3e-4a03-975c-6825e42d9163, status: active
inlets PRO TCP (0.8.3) server summary:
  IP: 51.158.189.204
  Auth-token: JoWxVv7yBuOH7BEEEZIzItd9EPENIifZtAIHJWe11n6oMSFN797wy3bXroRpzSxG

Connect client:

$ inlets-pro tcp client --url "wss://51.158.189.204:8123" \
>   --token "JoWxVv7yBuOH7BEEEZIzItd9EPENIifZtAIHJWe11n6oMSFN797wy3bXroRpzSxG" \
>   --upstream $UPSTREAM \
>   --ports $PORTS
2021/07/19 21:37:10 Starting TCP client. Version 0.8.5 - 8db64ded51b1455cf4ad027c52bc9cfbeb55c4b3
2021/07/19 21:37:10 Licensed to: Johan Siebens <redacted>, expires: 72 day(s)
2021/07/19 21:37:10 Upstream server: localhost, for ports: 8000
inlets-pro client. Copyright Alex Ellis, OpenFaaS Ltd 2020
INFO[2021/07/19 21:37:10] Connecting to proxy                           url="wss://51.158.189.204:8123/connect"
INFO[2021/07/19 21:37:10] Connection established.. OK.               

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
• [ ] added unit tests

[alexellis]

cc @dirien