Supporting both Python 2 and Python 3 with six
Supporting both Python 2 and Python 3 with six

As we know python3 will not support built-in basestring, so we
should use 'six' to support both python2 and python3.

Change-Id: I9bad381f175602322cb750d6c1b62beeee222cb2
Partial-Bug: #1249886

dims/ironic,varunarya10/ironic,openstack/ironic,naterh/ironic,ramineni/myironic,redhat-openstack/ironic,ionutbalutoiu/ironic,bacaldwell/ironic,Tan0/ironic,JioCloud/ironic,naototty/vagrant-lxc-ironic,hpproliant/ironic,pshchelo/ironic,dims/ironic,bacaldwell/ironic,debayanray/ironic_backup,naototty/vagrant-lxc-ironic,devananda/ironic,supermari0/ironic,openstack/ironic,faizan-barmawer/openstack_ironic,ionutbalutoiu/ironic,SauloAislan/ironic,pshchelo/ironic,NaohiroTamura/ironic,SauloAislan/ironic,rdo-management/ironic,rackerlabs/ironic,NaohiroTamura/ironic,froyobin/ironic,Tehsmash/ironic