Apache + PHP performance tuning

Php's memory_limit is calculated in a `shell` task, and the rest are calculated in a `template` of an apache include (implemented using a custom `replace` module that's currently a PR for ansible core; see ansible/ansible#4889).

The following global vars can be specified as manual overrides for above calculations:

* `php_memory_limit`
* `reserved_memory_mysql`
* `reserved_memory_system`
* `apache_start_servers`
* `apache_min_spare_servers`
* `apache_max_spare_servers`
* `apache_max_clients`
* `apache_max_requests_per_child`

Fixes #56