Allow for custom per-vhost log_format

This is similar in spirit to the changes in #148 but with per-vhost log
formats.

It depends on the user creating the log_format in the
$vhost_cfg_prepend hash and it disalbes itself if the $access_log
parameter is set.

An example:
```puppet
  nginx::resource::vhost { 'custom.log.format':
    ensure   => present,
    vhost_cfg_prepend => {
      'log_format' => 'json \'{"timestamp": "$time_iso8601","remote_addr": "$remote_addr",...etc...}\'',
    },
    access_log_format => 'json',
  }
```
Add vhost_cfg_prepend to ssl, sort the (pre|ap)pend

SSL vhosts now get the $vhost_cfg_prepend inserted in the
header.

Both the prepend and append options are sorted to avoid rebuilding
config files when not module changes have been made.