Bug 1469435

As an operator, I want to trigger a Swift ring rebalance without executing a full overcloud update. 

Today rebalances are only executed if there is a change in the devices (IP address, device added etc). It is executed during an overcloud update. This is a huge overhead if one only wants to rebalance the rings. To make things worse, rebalances might need to be executed multiple times - there should be at least an option to automate this.

Some recent changes in Tripleo/Mistral make it possible to execute Ansible playbooks within a workflow. Implementing the rebalance as a workflow has multiple benefits:

1. It can be executed by an operator as a standalone operation on the undercloud
2. It can be executed automatically by Mistral multiple times (eg when there is a bigger rebalance required)

The workflow itself could be nearly the same as today:

1. Download most recent rings from the undercloud
2. Rebalance
3. Upload updated rings to the overcloud

It could be also done in a more lightweight manner, for example the above workflow is executed only on a single node, and once the updated rings are available they will be fetched by all nodes.

There should be an additional sanity check that uses the swift-dispersion-report tool to check if the dispersion already reached a given level (for example 95%) and also ensure that at least one replication pass finished since the last rebalance (using swift-recon data).

Expected outcome
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Operator can trigger a Mistral workflow that rebalances Swift rings in a safe manner and distributes them. 

Work items
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1. Add option to swift-ring-builder to limit rebalance operations to a maximum of X percent
2. Make swift-dispersion-report importable
3. Write Ansible playbook & library to execute descibed workflow