roles/php-fpm: Support php-fpm 7.2 on Ubuntu 18.04

Adds a separate handler for the php-fpm7.2 service and cleans up the
pool config file to remove some content that causes Jinja2 to choke.

roles/php-fpm/templates/php7.2-pool.conf.j2

@@ -301,13 +301,9 @@ pm.max_spare_servers = 3
 ;  %t: server time the request was received
 ;      it can accept a strftime(3) format:
 ;      %d/%b/%Y:%H:%M:%S %z (default)
-;      The strftime(3) format must be encapsuled in a %{<strftime_format>}t tag
-;      e.g. for a ISO8601 formatted timestring, use: %{%Y-%m-%dT%H:%M:%S%z}t
 ;  %T: time the log has been written (the request has finished)
 ;      it can accept a strftime(3) format:
 ;      %d/%b/%Y:%H:%M:%S %z (default)
-;      The strftime(3) format must be encapsuled in a %{<strftime_format>}t tag
-;      e.g. for a ISO8601 formatted timestring, use: %{%Y-%m-%dT%H:%M:%S%z}t
 ;  %u: remote user
 ;
 ; Default: "%R - %u %t \"%m %r\" %s"