Teach periodicCleanup how to do one provider.
Teach periodicCleanup how to do one provider.

This permits (once the cron changes are wired in) per-provider
threads, rather than a single global thread.

Change-Id: Ie666d03ee9493373de494c3e6664e4e7bf144dcb

seandst/nodepool,citrix-openstack/nodepool,Tesora/tesora-nodepool,seandst/nodepool,Tesora/tesora-nodepool,citrix-openstack/nodepool