eoptasks: add flag to list selected servers

eoptasks/eoptasks.py

@@ -58,8 +58,9 @@ for group in servergroups:
         servers.append(Server(servername, group))
 
 parser = argparse.ArgumentParser()
+parser.add_argument('-l', '--list-servers', action='store_true')
 parser.add_argument('-k', dest='keywords', type=str)
-parser.add_argument('cmd', type=str)
+parser.add_argument('cmd', type=str, nargs='?', default=None)
 
 args = parser.parse_args()
 
@@ -73,9 +74,19 @@ if args.keywords:
 else:
     selected_servers = servers
 
-if not selected_servers:
+if args.list_servers:
+    for server in sorted(selected_servers, key=lambda x: x.name):
+        print(server.name)
     sys.exit(0)
 
+if not selected_servers:
+    sys.stderr.write('No matching servers\n')
+    sys.exit(1)
+
+if not args.cmd:
+    sys.stderr.write('Missing command\n')
+    sys.exit(1)
+
 cmd = {
     'apt.update': 'sudo apt update',
     'apt.upgrade': 'sudo apt update && sudo apt upgrade -y',