Support specifying custom log formats

The goal being to support custom logging formats which must be declared before their usage in the access_log directive. However before I finalize this and write tests I started wondering if there would be value in making log_format a parameter in the nginx class. I'm not sure if anyone would ever define multiple log formats in which case a log_formats parameter might make sense. Any and all feedback welcome :smile: 

`````
nginx::http_cfg_prepend:
  log_format: 
  |
    custom '[$time_local] $remote_addr "$http_x_forwarded_for" $http_host '
           '"$request" $status $body_bytes_sent '
           '"$http_referer" "$http_user_agent" "$upstream_addr" '
           '"$upstream_http_x_alpha" "$upstream_http_x_beta"'
nginx::http_access_log: '/var/log/nginx/access.log custom'
`````