vnfdgen: replace six.iteritems() with .items()

vnfdgen: replace six.iteritems() with .items()

Openstack python3 style guide prefers to not use six.iteritems() but
instead use data.items() for all cases.

We should follow the Openstack guides since OPNFV is aligned with
Openstack

https://wiki.openstack.org/wiki/Python3#Port_Python_2_code_to_Python_3

Common patterns:
* Replace "for key in dict.iterkeys()" with "for key in dict"
* Replace dict.iteritems() with dict.items()
* Replace dict.itervalues() with dict.values()

Change-Id: I99b0505199a568771341658caf8b71c1896db9cd
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>