--force flag is ignored for trusted clusters

[kontsevoy]

Here's what I get when I try to create a trusted cluster twice:

$ sudo tctl create -f /var/lib/teleport/trusted-cluster.yaml 
A fatal error occurred: trusted cluster is already enabled

In this case --force is basically the same as "update". The reason this flag is so important is because it allows you to create deployment scripts that run multiple times i.e. you want to ignore "already exists" situations without having to inject conditionals like "if it exists do not do that".

If it's enabled and a user wants to enable it -> just let it remain enabled. No need to bark with fatal errors.