Implements host uptime API call for cell setup.

Fixes bug os-hypervisors/<id>/uptime fails in cell setup.Implements
'get_host_uptime' for cells_api, which returns hypervisor uptime
from the target child cell.

Fixes bug LP 1192911

Change-Id: Iac75e46e565f17c91117dcb810f6d2f2e62708d8
Merge "Implements host uptime API call for cell setup."