Prefix private methods with _ in docker driver

Private methods should be prefixed with a _ for
clarity. Although most private methods were
marked this way, a few were not.

Renames and marks private:
 is_daemon_running
 find_container_by_name
 get_available_resource

Closes-Bug: #1267664

Change-Id: Ifc6818b7620af972cc0eec6f9ef43610456895f1
Merge "Prefix private methods with _ in docker driver"