Allow overriding the detected $apache_name.

This will mainly be useful for supporting Red Hat Software Collections, where 'httpd24-httpd' installs Apache 2.4 alongside Apache 2.2 in the base repository. The $service_name parameter already allows customization of the service to use, but not the package.

With this PR, the following can be used to set up and run Apache 2.4 on RHELish systems. (The config snippet I mentioned in #818 was missing this.)

```puppet
  softwarecollectionsorg { 'httpd24': }
  class { '::apache':
    apache_name    => 'httpd24-httpd',
    service_name   => 'httpd24-httpd',
    apache_version => '2.4',
    mpm_module     => 'event',
    httpd_dir      => '/opt/rh/httpd24/root/etc/httpd',
    vhost_dir      => '/opt/rh/httpd24/root/etc/httpd/conf.d',
    require        => Softwarecollectionsorg['httpd24'],
  }
```