Clear drush cache after installation

debian/aegir3-hostmaster.postinst

@@ -140,6 +140,9 @@ case "$1" in
   "client_email": "$EMAIL"
 }
 EOF
+            # flush the drush cache to find new commands
+            su -s /bin/sh aegir -c 'drush cache-clear drush'
+
             # on new installs, we default to having the daemon enabled
             echo 'Enabling hosting-queued daemon'
             su -s /bin/sh aegir -c 'drush @hostmaster pm-enable -y hosting_queued'