Use time.sleep() instead of eventlet.sleep()

In a patched environment, time.sleep will be eventlet.sleep, so this
does not have any actual impact, but it removes one direct dependency on
eventlet.

Change-Id: I1603cfa94c44c9fdd4c79e332ca981f6c9f05625

manilaclient/common/httpclient.py

@@ -28,10 +28,7 @@ import requests
 
 from manilaclient import exceptions
 
-try:
-    from eventlet import sleep
-except ImportError:
-    from time import sleep  # noqa
+from time import sleep  # noqa
 
 try:
     osprofiler_web = importutils.try_import("osprofiler.web")