Improve error message when cluster.crm_from is invalid

If a user passes a non-date string value as the crm_from parameter
of the cluster plugin an error message is logged:

  crm_from parameter 'True' is not a valid date

The plugin continues to run and uses the default value (T-72hrs)
as the value of crm_from. Make this clear in the message displayed
to users:

  crm_from parameter 'True' is not a valid date: using default

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>