Import osapi_v3/enabled option in nova/test

Since `nova/api/openstack` may not have been imported by the time we use this
option in `nova/test`, we need to explicitly import it at the top of the file.

Fixes bug 1187847

Change-Id: Ibc9f3466d3baaf47d44bda2fc22247a494809909
Merge "Import osapi_v3/enabled option in nova/test"