Pull the primary host from the role

So commands like this run on a host with the specified role:
```
mrsk app exec -r=console -i "/bin/bash`
mrsk app logs -f -r=workers
```

lib/mrsk/commander.rb

@@ -35,7 +35,7 @@ class Mrsk::Commander
   end
 
   def primary_host
-    specific_hosts&.first || config.primary_web_host
+    specific_hosts&.first || specific_roles&.first&.primary_host || config.primary_web_host
   end
 
   def roles