Make sure that shared drush command folder is present

What about including this in init.pp, like @tizzo has done at @zivtech:
https://github.com/zivtech/vagrant-development-vm/blob/master/puppet-modules/drush/manifests/init.pp#L29-L42

```puppet
  file { "/usr/share/drush":
    ensure => directory,
    owner => root,
    group => root,
    mode => 755,
  }

  file {  "/usr/share/drush/commands":
    require => File['/usr/share/drush'],
    ensure => directory,
    owner => root,
    group => root,
    mode => 755,
  }
```

Still new to puppet, but I think the rationale is so that globally-installed drush commands can depend on the drush module without redeclaring:
https://github.com/zivtech/vagrant-development-vm/blob/master/puppet-modules/drushfetcher/manifests/init.pp#L1-L4