Fix broken glance-cache-prefetcher utility

glance-cache-preftecher utility was failing because it was not
setting global threadpool model while initilizing the tool.

Closes-Bug: #2065087
Change-Id: I0c09dd7c0d81178ebf2aa5efd19c54866fc8791b
(cherry picked from commit 32ee57c)
(cherry picked from commit 99540a9)
(cherry picked from commit cec35e9)

Author: konan-abhi

glance/cmd/cache_prefetcher.py

@@ -36,6 +36,7 @@
 import glance_store
 from oslo_log import log as logging
 
+import glance.async_
 from glance.common import config
 from glance.image_cache import prefetcher
 
@@ -49,6 +50,7 @@ def main():
         config.parse_cache_args()
         logging.setup(CONF, 'glance')
         CONF.import_opt('enabled_backends', 'glance.common.wsgi')
+        glance.async_.set_threadpool_model('eventlet')
 
         if CONF.enabled_backends:
             glance_store.register_store_opts(CONF)