Template for /etc/default/varnish

Uses memory, size set by node[:magento][:varnish][:memory]
MySQL inserts to configure PageCache module

Variable 'varnishservers' should be 'localhost' or semicolon-separated list of front-end IPs.
TTL increased from default, everything else the same.
Update varnish.erb
Enable PageCache with SQL inserts from template

Also added "10.0.0.0"/8 to acl purge, for easier scaling.
Include firewall HTTP port in Varnish template
Use template for all Varnish config

Removed various sed commands in favour of template use.
Config works on CentOS and Ubuntu, with different file location.
Added Varnish memory config variable

Value is 1/4 of node's memory
Syntax fix for firewall HTTP port substitution
Merge pull request #5 from will-parsons/master

Varnish config template and PageCache enabling