Tests for os-hypervisors API extension of Nova

Tests against following paths are added:
os-hypervisors
os-hypervisors/detail
os-hypervisors/%hyper_id
os-hypervisors/%hyper_name/servers
os-hypervisors/statistics
os-hypervisors/%hyper_id/uptime

Implements: blueprint nova-os-hypervisors-api-extension-test
Change-Id: Ia209b113d0193720b445385e0fe399b81f48bb4b
(cherry picked from commit 3d5f16331cc1a88479fc60853ea08533775a3b38)