Add the ability to quickly shutdown one or all pools

[bmr-cymru]

In some circumstances (for e.g. testing when creating/tearing down configurations repeatedly on the same storage) it would be useful to be able to shut down either a specific Stratis pool, or all pools present on the system. This would release the associated device-mapper mappings and allow the device content to be erased with tools like wipefs(8).

At the pool level this could look like this in the stratis-cli:

# stratis pool disable <name>

On success the devices previously used by the pool should be unused.

A corresponding "enable" verb (or "stop/start", "enable/shutdown" etc.) may also be helpful for some users although it's less important for my current testing needs.

[jbaublitz]

@mulkieran Are we considering this the main issue for starting and stopping pools?