Allow user to control metric frequency

[ofaaland]

The cerebro_module_devel(3) man page says:

The 'get_metric_period' function retrieves the period in seconds that the metric value should be read and propogated. If the period is 0, the metric will be read and propagated with every cerebrod heartbeat.

Currently all the LMT metrics modules (ie lmt_mdt) have get_metric_period() functions which return LMT_UPDATE_INTERVAL which is a macro defined to be "5".

This could probably be configurable.

It could certainly be documented better, so users know why setting cerebrod_heartbeat_frequency in cerebro.conf doesn't have any impact.