Use correct site name for purge-varnish log and pid

Allows the log and pid file names to be set dynamically by the
config_files:convert_init_script rake task.

ENV[‘SITE’] can be specified to name the particular install – e.g.
rather than 'alaveteli' you might name the site 'foi' – so now the log
and pid files are created with the correct name rather than simply
'purge-varnish'
Ensure the log and pid directories exist

If the directories do not exist the daemon cannot start.