setup nginx for load balancing, setting HOME variable in unicorn_init, and fix for missing nginx log directory when web is a non-release role

- (temporary?) fix to problem in unicorn_init script whereby HOME variable is not set (possible alternative to https://github.com/bruno-/capistrano-unicorn-nginx/pull/11)
- create log directory for nginx if web is a non-release role
- connect over TCP/IP if there are multiple app roles and setup nginx for load balancing
I noticed `unicorn_tcp_listen_port` option default  is defined in `unicorn.rake` file so I think the definition here is redundant.