opcode summary: improve display for list summaries

Currently, opcodes like NODE_EVAC_STRATEGY look bad:

  89684 error   NODE_EVAC_STRATEGY([u'node3'])

With this patch, we try to render list arguments a little bit better:

  89684 error   NODE_EVAC_STRATEGY(node3)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>