Spawn() method signature mismatches with latest kilo release codes

Consider after a while this "instance_type" has been changed to "flavor",
and we never use them, to avoid frequently update the powervc-driver
signature, append "**kwargs" to spawn() method.

Change-Id: I44c664c4e1650955e1eaa1f619223c00ade6f999
Closes-Bug: 1403378
Merge "Spawn() method signature mismatches with latest kilo release codes"