Read /etc/crowbar.install.key when CROWBAR_KEY env var is not set

There's no reason the tools cannot try to read the file when the
environment variable is not set.
I agree with @aspiers, I think that ```warn``` + ```exit``` in the rescue clause would be better:

```ruby
begin
  key = File.read(@crowbar_key_file).strip
rescue
  warn "Unable to read crowbar key from #{@crowbar_key_file} or from CROWBAR_KEY env variable."
  exit 1
end
```