stats_cron_schedule description is misleading

[Shane Peters]

The description for this charm option is confusing. Leaving it 'unset' implies a default value which *does* enable the cron task.

 stats_cron_schedule:
      type: string
      default: '*/5 * * * *'
      description: |
        Cron schedule used to generate rabbitmq stats. If unset
        no stats will be generated.

This should be revised to say that setting an empty value will disable the cron task.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/473552

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/473552
Committed: https://git.openstack.org/cgit/openstack/charm-rabbitmq-server/commit/?id=01386e0cb2d73f3e375d841d3a85a8001addb7db
Submitter: Jenkins
Branch: master

commit 01386e0cb2d73f3e375d841d3a85a8001addb7db
Author: Shane Peters <email address hidden>
Date: Mon Jun 12 14:23:04 2017 -0400

    Clarify description of stats_cron_schedule option

    The description for this option is confusing. Leaving it unset
    implies using the default value which enables the cron task.

    This change clarifies to the user they must actually set the value to
    an empty string in order to disable the cron task.

    Change-Id: I39efdfe574412ec5a188794b32b5cf221c40e884
    Closes-Bug: #1697523