Drush 9 support

Our Jenkins test with the master branch from Drush (which is working towards 9.x) fails. The hostmaster platform looks properly build at that time.

Ref: http://ci.aegirproject.org/job/P_Aegir_Puppet_Module_functional_test_Aeg...

The error we log there to install.log:

 [debug] Returned from hook drush_make [18.95 sec, 12.66 MB]
 [info] Command dispatch complete [18.96 sec, 12.63 MB]
 [error]  We could not find an applicable site for that command. [18.98 sec, 12.55 MB] 
 [error]  Drush could not bootstrap this platform. Please check the platform directory exists and is readable. [18.98 sec, 12.56 MB] 
 [debug] Returned from hook drush_provision_drupal_pre_provision_verify [18.98 sec, 12.23 MB]
 [info] Command dispatch complete [18.98 sec, 12.19 MB]
 [debug] Caught drush error, ending drush_provision_hostmaster_install - B [18.99 sec, 12.47 MB]
 [debug] Array
(
    [DRUSH_NO_SITE] => Array
        (
            [0] => We could not find an applicable site for that command.
        )

    [DRUSH_INVALID_DRUPAL_ROOT] => Array
        (
            [0] => Drush could not bootstrap this platform. Please check the platform directory exists and is readable.
        )

)
 [18.99 sec, 12.47 MB]
 [debug] Returned from hook drush_provision_hostmaster_install [18.99 sec, 12.47 MB]
 [info] Command dispatch complete [18.99 sec, 12.43 MB]