Feature: Make memory_limit configurable for php.ini file

[mamedin]

The memory_limit is hardcoded to 512M here (CentOS/RHEL only):

https://github.com/artefactual-labs/ansible-atom/blob/master/tasks/php-rh.yml#L56

When generating a Finding Aid for a large Fond, it fails when using the default memory_limit=512M:

[info] [2020-11-09 02:29:43] Job 597029 "arFindingAidJob": Job started.
[info] [2020-11-09 02:29:43] Job 597029 "arFindingAidJob": Generating finding aid (doukhobor-collection)...
[info] [2020-11-09 02:30:36] Job 597029 "arFindingAidJob": Exporting EAD has failed.
[info] [2020-11-09 02:30:36] Job 597029 "arFindingAidJob": ERROR(EAD-EXPORT): PHP Warning:  count(): Parameter must be an array or an object that implements Countable in /usr/share/nginx/atom/plugins/sfEadPlugin/modules/sfEadPlugin/templates/indexSuccessBodyDidElement.xml.php on line 137
[info] [2020-11-09 02:30:36] Job 597029 "arFindingAidJob": ERROR(EAD-EXPORT): PHP Warning:  count(): Parameter must be an array or an object that implements Countable in /usr/share/nginx/atom/plugins/sfEadPlugin/modules/sfEadPlugin/templates/indexSuccessBodyDidElement.xml.php on line 137
[info] [2020-11-09 02:30:36] Job 597029 "arFindingAidJob": ERROR(EAD-EXPORT): PHP Warning:  count(): Parameter must be an array or an object that implements Countable in /usr/share/nginx/atom/plugins/sfEadPlugin/modules/sfEadPlugin/templates/indexSuccessBodyDidElement.xml.php on line 137
[info] [2020-11-09 02:30:36] Job 597029 "arFindingAidJob": ERROR(EAD-EXPORT): PHP Warning:  count(): Parameter must be an array or an object that implements Countable in /usr/share/nginx/atom/plugins/sfEadPlugin/modules/sfEadPlugin/templates/indexSuccessBodyDidElement.xml.php on line 137

But increasing to 2048M it worked.