Manage /etc/php5/mods-available recursively

* Prefix the file name of Puppet-managed ini files with an underscore.
* Exclude files not starting with an underscore from being managed recursively.
* Remove ensure => absent option from php::ini (simply drop the resource to
  have it cleaned up).

Bug: 55604
Change-Id: If17f43ca74aad17dbecf7b9bab99124d6dd84306
Merge "Manage /etc/php5/mods-available recursively"