python3: compatibility for iteritems and iterkeys

Use six to allow python2/pyton3 for iteritems and
iterkeys.

six.iteriems() replaces dictionary.iteritems() (python2)
and dictionary.iterms() (python3)

six.iterkeys() replaces dictionary.iterkeys (python2)
and dictionary.keys() (python3)

Change-Id: I26c80b78a7dedf3aa32eedf01a83ff6d1e592ba7
Signed-off-by: Chuck Short <chuck.short@canonical.com>