Add default to distribute option

Author: ekouts

docs/manpage.rst

@@ -379,7 +379,7 @@ Options controlling ReFrame execution
 
    .. versionadded:: 3.2
 
-.. option:: --distribute=NODESTATE
+.. option:: --distribute[=NODESTATE]
 
    Distribute the selected tests on all the nodes in state ``NODESTATE`` in their respective valid partitions.
 
@@ -397,6 +397,8 @@ Options controlling ReFrame execution
      The states of the nodes will be determined once, before beginning the
      execution of the tests so it might be different in the time of the submission of the tests.
 
+   If ``NODESTATE`` is not passed it will take ``idle`` as default.
+
    .. versionadded:: 3.11.0
 
 .. option:: --exec-policy=POLICY

reframe/frontend/cli.py

@@ -372,9 +372,10 @@ def main():
         default=[], help='Disable a pipeline hook for this run'
     )
     run_options.add_argument(
-        '--distribute', action='store', default=None, metavar='{all|STATE}',
+        '--distribute', action='store', metavar='{all|STATE}',
+        nargs='?', const='idle',
         help=('Distribute the selected single-node jobs on every node that'
-              'is in STATE')
+              'is in STATE (default: "idle"')
     )
     run_options.add_argument(
         '--exec-policy', metavar='POLICY', action='store',