Concatenate config_file and config_dir in server erb variable

The @config_file variable used in init.logstash_server.erb points to the config directory but does not include the name of the file itself (node['logstash']['server']['config_file']).  Before I made this change the generated script would ignore any of our config changes. 

I believe the same issue occurs in agent.rb and logstash_agent.conf.erb but we have not tested it.