Add small cluster thrashing tasks

Signed-off-by: Samuel Just <samuel.just@dreamhost.com>

suites/regression/thrash/btrfs/with-btrfs.yaml

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+overrides:
+  ceph:
+    btrfs: 1

suites/regression/thrash/clusters/6-osd-2-machine.yaml

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+roles:
+- [mon.0, mds.a, osd.0, osd.1, osd.2]
+- [mon.1, mon.2, client.0, osd.3, osd.4, osd.5]

suites/regression/thrash/thrashers/default.yaml

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+tasks:
+- ceph:
+    log-whitelist:
+    - wrongly marked me down or wrong addr
+- thrashosds:

suites/regression/thrash/workloads/radosbench.yaml

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+tasks:
+- radosbench:
+    clients: [client.0]
+    time: 1800

suites/regression/thrash/workloads/readwrite.yaml

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+tasks:
+- rados:
+    clients: [client.0]
+    ops: 4000
+    objects: 500
+    op_weights:
+      read: 45
+      write: 45
+      delete: 10

suites/regression/thrash/workloads/snaps.yaml

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+tasks:
+- rados:
+    clients: [client.0]
+    ops: 4000
+    objects: 500
+    op_weights:
+      read: 100
+      write: 100
+      delete: 50
+      snap_create: 50
+      snap_remove: 50
+      snap_rollback: 50