rptest: Wait for Stable before start cgrups tests

- Its been observed many times in the ConsumerGroupTests that an
unbalanced consumer consumes all messages and and some type of assertion
that expects at least all members to consume some records fails by way
of timing out.

(cherry picked from commit c912d69)

tests/rptest/tests/consumer_group_test.py

@@ -91,7 +91,7 @@ def create_consumers(self,
 
         def group_is_ready():
             gr = rpk.group_describe(group=group, summary=True)
-            return gr.members == consumer_count
+            return gr.members == consumer_count and gr.state == "Stable"
 
         wait_until(group_is_ready, 60, 1)
         return consumers