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.

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

Closes-bug: #1347777
(cherry picked from commit b856bc5e348cc69923e3ee815303804323730026)

Change-Id: Ic7e16fd5154609987b2ef0b6f8ee52619ccf489c
Merge "Add Hyper-V driver in the "compute_driver" option description" into stable/havana