Apache + PHP performance tuning

Php's memory_limit is calculated in a `command` 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_settings.memory_limit`
* `reserved_memory.mysql`
* `reserved_memory.system`
* `apache_settings.start_servers`
* `apache_settings.min_spare_servers`
* `apache_settings.max_spare_servers`
* `apache_settings.max_clients`
* `apache_settings.max_requests_per_child`

Fixes #56