Consolidate httpd/apache2 Service and Package

Since both the 'httpd' and 'apache2' variants of the package and service are
now defined in modules/apache/manifests/init.pp, it's easy to see that any
manifest that references 'httpd' should be able to reference 'apache2'.
So update all references to 'apache2' name and remove the 'httpd' variants.
The service is still dummy for now, as a precaution.

Change-Id: Iefab367303ff03c3ae16a70568e7c6176e122147