Allow use with multiple server Trellis

[dalepgrant]

Overview

Using this role as-is on a Trellis setup with multiple servers is not possible because it fails when no license key is set. This PR will only run install tasks if a license key is set, but allow provisioning to complete if it isn't.

Use case

We're using Trellis with multiple servers, set up under host_vars e.g.

trellis
    └── host_vars/ 
        ├── server1/ 
        └── server2/ 
            ├── wordpress_sites.yml/
            └── vault.yml/  

In a simplified example, we want to use New Relic on server 2 but NOT server 1. By allowing a false value to be set (or defaulting to it), the New Relic install tasks can be skipped for servers we don't want to install it on.

Feedback welcome and thanks for making this role available in the first place!

[dalepgrant]

@tangrufus Happy New Year! Just politely bumping this to see if it's something worth merging in. Happy to make changes if necessary.