print usage when no argument is specified for python3

When running just 'glance' under python3, we will get the error:
ERROR: 'Namespace' object has no attribute 'func'

This is because map() is used to decode sys.argv, but under Python3
it returns a map object which is an iterable. Some code later tries
to use this in a boolean context and it will always return True,
even if it's empty.

Change-Id: I2f03e462cb813833b75b9f2de7badd10b10cddff
Closes-Bug: #1295356

glanceclient/shell.py

@@ -759,7 +759,8 @@ class HelpFormatter(argparse.HelpFormatter):
 
 def main():
     try:
-        OpenStackImagesShell().main(map(encodeutils.safe_decode, sys.argv[1:]))
+        argv = [encodeutils.safe_decode(a) for a in sys.argv[1:]]
+        OpenStackImagesShell().main(argv)
     except KeyboardInterrupt:
         utils.exit('... terminating glance client', exit_code=130)
     except Exception as e: