Add Hyper-V driver in the "compute_driver" option description

The description of the option "compute_driver" should include
hyperv.HyperVDriver along with the other supported driver.

Change-Id: Ic7e16fd5154609987b2ef0b6f8ee52619ccf489c
Closes-bug: #1347777

Conflicts:
	nova/virt/driver.py
	nova/virt/hyperv/__init__.py