tasks/thrashosds: support overrides

e.g.,

overrides:
  thrashosds:
    thrash_primary_affinity: false
...
tasks:
- install:
- ceph:
- thrashosds:
- workunit:
...

Needed for #9865

Signed-off-by: Sage Weil <sage@redhat.com>

tasks/thrashosds.py

@@ -112,6 +112,8 @@ def task(ctx, config):
         config = {}
     assert isinstance(config, dict), \
         'thrashosds task only accepts a dict for configuration'
+    overrides = ctx.config.get('overrides', {})
+    teuthology.deep_merge(config, overrides.get('thrashosds', {}))
 
     if 'powercycle' in config: