ducktape: GroupMetricsTest check if leader exists

[ZeDRoman]

Cover letter

In failure attached by Lena here: #3394 (comment)
I found problem with leader check.

In group description Redpanda returns {'ns': 'kafka_internal', 'topic': 'group', 'partition_id': 0, 'status': 'done', 'leader_id': -1, 'raft_group_id': 8, 'replicas': [{'node_id': 3, 'core': 0}, {'node_id': 2, 'core': 0}, {'node_id': 1, 'core': 0}]}
Where leader_id is -1. self.redpanda.get_node(leader) == new_leader accepts this.

This happens because self.redpanda.get_node(idx) returns this self.nodes[idx - 1]

Fixes #3394

[NyaliaLui]

Excellent find.