Do not block the event loop with time.sleep in the async clients

nodediscover's rescan poll and nodeconsole's screenshot refresh both slept
with time.sleep inside a coroutine.  In nodeconsole --video that stops the
input handler and the VNC streaming tasks for the whole interval.

confluent_client/bin/nodeconsole

@@ -910,7 +910,7 @@ async def do_screenshot():
             dorefresh = False
         else:
             dorefresh = True
-            time.sleep(options.interval)
+            await asyncio.sleep(options.interval)
     sys.exit(0)
 
 async def grab_vncs(urlbynode):

confluent_client/bin/nodediscover

@@ -20,7 +20,6 @@ import csv
 import optparse
 import os
 import sys
-import time
 
 path = os.path.dirname(os.path.realpath(__file__))
 path = os.path.realpath(os.path.join(path, '..', 'lib', 'python'))
@@ -364,7 +363,7 @@ async def assign_discovery(options, session, needid=True):
 async def blocking_scan(session):
     list([x async for x in session.update('/discovery/rescan', {'rescan': 'start'})])
     while(list([x async for x in session.read('/discovery/rescan')])[0].get('scanning', False)):
-        time.sleep(0.5)
+        await asyncio.sleep(0.5)
     list([x async for x in session.update('/networking/macs/rescan', {'rescan': 'start'})])