Support Drush 7.x

We need to be able to support Drush 7 sooner, rather than later. The reason we went with Drush 6 was largely due to the decision to ban Drupal 8 support in Drush 7. However, we're going to be stuck moving to Drush 8 in order to properly support Drupal 8 anyway. This'll entail upping out PHP requirements to PHP 5.5.9+, which'll break support for Drupal 6.

I propose that we move to Drush 7 as soon as possible. Then start working on Drush/Drupal 8 support again. I believe the combinations we'll need support will look like:

• Aegir3 + Drush7 = Drupal 6/7
• Aegir3 + Drush8 = Drupal 7/8

Since we install Drush using composer directly in our .debs now, we'll need to add a parameter to set which Drush version to install. We can stick with Drush6 for now, and let folks opt-in to Drush 7, and eventually 8.

I've created a new test for Aegir 3 on Drush 7: http://ci.aegirproject.org/job/P_Aegir_Puppet_Module_functional_test_Aeg...

As expected, this test is failing. I did fix up puppet-drush and puppet-aegir to allow us to get all the way to "drush hostmaster-install" without errors on Debian Jessie. From the install log, it looks like Drush is trying to connect to localhost via SSH.