Use six.moves.map instead of itertools.imap

In Python 3, module "itertools" doesn't contain "imap" object.
To support both Python 2 and Python 3, "six.moves.map"
should be used instead.

Change-Id: I5b82020d0721a91aa6d6090a42073a3285b80ff9
Closes-Bug: #1403434