plugins: add btrfs plugin

[0x666c697473]

Resolves: #840

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[bmr-cymru]

The CI failure is due to code formatting issues - run black snapm and it will fix those up.

[bmr-cymru]

Need to add the new plugin file to the RPM files list.. that's what's up with the RPM builds.

[bmr-cymru]

Oh also this:

=================================== FAILURES ===================================
_____________________ LoaderTestsSimple.test_load_plugins ______________________

self = <tests.test_loader.LoaderTestsSimple testMethod=test_load_plugins>

    def test_load_plugins(self):
        plugin_classes = load_plugins()
>       self.assertEqual(len(plugin_classes), 3)
E       AssertionError: 4 != 3

There's a slightly silly thing in the test suite where we hard code the number of plugins in test_load_plugins; I should fix that properly, but for now it's fine to just s/3/4 in teh test case.