tests/clients: summary mode for describe group

The default mode is rather expensive for high partition counts,
and complicates handling systems in transient states when one
or more of the partitions is likely to be underoing leadership
movement and therefore have NOT_LEADER errors etc in the
default per-partition output.

When all we want to know is the group's state, this lets
us get that.

tests/rptest/clients/rpk.py

@@ -339,7 +339,7 @@ def group_seek_to(self, group, to):
         cmd = ["seek", group, "--to", to]
         self._run_group(cmd)
 
-    def group_describe(self, group):
+    def group_describe(self, group, summary=False):
         def parse_field(field_name, string):
             pattern = re.compile(f" *{field_name} +(?P<value>.+)")
             m = pattern.match(string)
@@ -365,6 +365,11 @@ def check_lines(lines):
                 # Leadership movements are underway
                 if 'NOT_LEADER_FOR_PARTITION' in line:
                     return False
+
+                # Cluster not ready yet
+                if 'unknown broker' in line:
+                    return False
+
             return True
 
         def parse_partition(string):
@@ -399,7 +404,11 @@ def parse_partition(string):
                                      host=m['host'])
 
         def try_describe_group(group):
-            cmd = ["describe", group]
+            if summary:
+                cmd = ["describe", "-s", group]
+            else:
+                cmd = ["describe", group]
+
             try:
                 out = self._run_group(cmd)
             except RpkException as e: