Calculating php memory_limit and apache prefork directives; allowing manual overrides as global vars

* if no manual override, calculating php memory_limit via `shell` task
* replacing apache default prefork config with include to separate file
* generating prefork.conf from template (incl calculated directive values)
* using custom `replace` module, see ansible/ansible#4889

Fixes #56
Replaced bash psuedo-ternary with jinja inline conditional
Switched to manual overrides grouped into sections; removed conflicting settings from roles/wordpress/vars

Note that template logic in prefork.conf is necessarily more complex (as we cannot assign to dicts), but also somewhat more explicit and less magical. I'm okay with this.
Fixed issue where php_settings, reserved_memory, or apache_settings are not defined
Removed outdated var names from common/vars/main
Merge pull request #64 from genesis/56-apache-php-tuning

Apache + PHP performance tuning