Add possibility of configure clean up period

[cardil]

Now clean up period is strict 10 minutes. In some cases it's a little long. It should be handy to be able to configure this period.

openstack-cloud-plugin/src/main/java/jenkins/plugins/openstack/compute/JCloudsCleanupThread.java

Lines 52 to 54 in 9df43ab

	public long getRecurrencePeriod() {
	return MIN * 10;
	}

[simonelbaz]

Done through #393