Support variables in servers

[aszenz]

Servers can have variables like

{    "servers": [
        {
            "url": "https://{tenant}.app.com",
            "description": "Production server",
            "variables": {
                "tenant": {
                    "default": "demo"
                }
            }
        },
        {
            "url": "https://{tenant}.staging.app.com",
            "description": "Staging server",
            "variables": {
                "tenant": {
                    "default": "demo"
                }
            }
        }
    ]
}

So when generating Servers.elm file support passing variables to server functions