Rename ironic option drivers_whitelist -> enabled_drivers

There were competing patches to implement this feature in Ironic.

Patch I2da37409cea98731a2ca18dfe7298861fde37783 calls the option
'enabled_drivers', which is more consistent with other option names.

Change-Id: Ifa7eae594c6d9ea743a17b4c31e407501bba8467
Merge "Rename ironic option drivers_whitelist -> enabled_drivers"