Support get_host_uptime in docker driver

The host uptime method is quite simple to support
and is low-hanging-fruit toward getting Tempest passing
and increasing the scope of API compatibility for the
Docker driver.

This change pulls get_host_uptime into a new hostutils
module using this method from both libvirt and docker.

Change-Id: I2e64af19c9849562390a232b3406eef235a369b8