Replace xrange() with six.moves.range()

Replace xrange() with six.moves.range()

Add "from six.moves import range" to replace the builtin range()
function with six.moves.range() to always create an iterator, instead of
creating a temporary list.

Replace "xrange" with "range".

Don't add the import for ranges of 1024 items or less.

Blueprint cinder-python3
Change-Id: If618b4e810e444f7eb6592bb2398805e9d14d548