Add 'python-six' dependency to nailgun package

Nailgun package had missing requirement for 'python-six' which was
causing usage of 'python-six' package in version that do not have
all required features.

Change-Id: Ieb9b18923fd6d797da09044c6a48f9228b64e910
Closes-Bug: #1366728
Remove dhcpcheck for juno support

Change-Id: I276b547a68e073738890560c5a57b3f008ce9902
Building OS images centos and ubuntu

Those images are supposed to be used for
image based provisioning. This patch set
needs python-imgcreate package to be available.

Implements: blueprint image-based-provisioning
Change-Id: I4f038c99d4caf7d2b6e291bff29572b3a45d92c7