rlib2: handle wait_for_sync in instance creation

Until now, wait_for_sync was harcoded to True when creating instances via RAPI.
We allow settings wait_for_sync via RAPI and have rlib2 honour this decision.

Note: this is not needed in the upcoming 2.5, as Opcode parameters are not
hardcoded in rlib2 anymore.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>