Use a defined environment variable in place of `apikey` if it exists

[kaihoffman]

Instead of requiring civo apikey save for every environment, have it so that the system will use the API key defined in an environment variable if it exists. This means you would be able to configure the env var for a CI/CD system once, download Civo CLI and have it automatically look at the defined variable for the appropriate key to use.

Suggested names for the variable:

• CIVO_APIKEY
• CIVO_TOKEN

(proposed by @rawkode)