Split the Node_addOffline test into pri/sec

Currently, this (single) test checks whether we can add an instance as
primary and secondary to a given node. This won't work for instance
templates other than DRBD8, so we split it into:

- an add to primary test, which should work for all disk templates
- an add to secondary test, for which we set the disk template to DRBD
  explicitly

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>