Fix 'spacing' parameters for periodic tasks

conductor.manager module contains 'spacing' parameters for periodic
tasks, they do not work because manager module imported before
parsing config options.
There is not such problem in Nova which uses create() method in class
derived from common Service for lazy loading manager module.
Because this derived class not needed for Ironic simple load_manager
function added to common.service.

Partial-Bug: #1279774
Change-Id: Ib31024c8eaf75d38de09983459d86125847cdf30
Merge "Fix 'spacing' parameters for periodic tasks"