Allow user to control metric frequency

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.

Start with the cerebro_module_devel(3) man page and the LMT metric modules' get_metric_period() implementations, especially their use of LMT_UPDATE_INTERVAL. Trace how cerebro.conf and cerebrod_heartbeat_frequency are currently handled. Done means the metric period can be configured as intended and the documentation explains its relationship to the heartbeat frequency.

Written by the indexing model from the issue text.