Allow overriding php_packages in play.

The suggested patch would make it possible to set php_packages in host_vars/group_vars etc. This is the only way I know of when using vars_include to support multiple distros. I imitated the naming style from jonhattan who uses it here: https://github.com/sbitmedia/ansible-fail2ban/commit/70a6b594a3d8e6789119751f675cfd593b6f2a3e

Would this be ok for merging? I contemplated about splitting the var to a one with the compulsory ones and other with optionals, but decided not to initially. The list of must haves would probably be rather short...