Add opcode attribute for comments

This attribute allows programmatic submitters of jobs (e.g. iallocator)
to add a comment to each opcode, describing its purpose. Example:

$ gnt-job info 123
Job ID: 123
  …
  Opcodes:
    OP_INSTANCE_REPLACE_DISKS
      …
      Input fields:
        comment: Replaces disks on inst1.example.com
      …

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