Add preparation section to Consul documentation

Breaks out steps for preparing to install consul. Adds a link to the
required ports for using Consul with GitLab

doc/administration/consul.md

@@ -17,9 +17,17 @@ a service networking solution that you can manage by using `/etc/gitlab/gitlab.r
 
 ## Configure the Consul nodes
 
-After you review the [reference architecture](reference_architectures/index.md#available-reference-architectures)
-documentation to determine the number of Consul server nodes you should have,
-on _each_ Consul server node:
+### Preparation
+
+1. Review the [reference architecture](reference_architectures/index.md#available-reference-architectures)
+documentation to determine the number of Consul server nodes you should have.
+1. If necessary, ensure the appropriate ports are open in your firewall.
+   See our [package documentation](https://docs.gitlab.com/omnibus/package-information/defaults.html#ports)
+   for the full list.
+ 
+### Installation
+ 
+On _each_ Consul server node:
 
 1. Follow the instructions to [install](https://about.gitlab.com/install/)
    GitLab by choosing your preferred platform, but do not supply the