Filter Nginx includes by sites present on target server

[dalepgrant]

One for those of us using multiple remote servers with Trellis.

Currently, Trellis templates out anything you put into nginx-includes/ to the remote servers. Take the following:

trellis/
└── group_vars/         
    └── production/
        ├── main.yml
        └── vault.yml
    └── staging/
└── host_vars
    └── server1/
        └── wordpress_sites.yml
    └── server2/
        └── wordpress_sites.yml
    └── staging1/
    └── staging2/

Assume server1 has a different site(s) to server2, and perhaps staging1 and staging 2 have additional sites. Your Nginx includes folder might look like this:

trellis/
└── nginx-includes/         
    └── all/
        ├── rewrites.conf.j2
        └── proxy.conf.j2
    └── site1/
        └── rewrites.conf.j2
    └── site2/
        └── rewrites.conf.j2
    └── site3/
        └── rewrites.conf.j2
...etc

All servers will end up with all Nginx includes files, even if the sites don't exist on that server:

/etc/nginx/includes.d/
└── all/  
    ├── rewrites.conf
    └── proxy.conf
└── site1/  
    └── rewrites.conf
└── site2/  
    └── rewrites.conf
└── site3/  
    └── rewrites.conf
└── site4/  
    └── rewrites.conf

This PR filters the Nginx includes that are to be 'included' by combining the sites that exist on the target server (taken from wordpress_sites keys) with a default 'all'. This follows with the default setup shown in the docs.

Also included is the ability to override the default 'all' folder. I've only included this because it is possible to override the block in the Nginx conf template and if someone did that, they may also want to determine their own Nginx includes locations.

To override or otherwise add to the Nginx includes locations, in e.g. group_vars/all/main.yml you can do

# single. will override default 'all' if changed. 
nginx_includes_extra_folders: all

# multiple
nginx_includes_extra_folders:
  - all
  - example
  - another

Note: in testing, the folders for sites not present on a remote are not cleaned up if they already exist but the files inside them are. That's to do with what happens when setting nginx_includes_d_cleanup: true which is a separate issue.