Removed unused iso8601 dependency
Removed unused iso8601 dependency

It should have been removed when migrating to oslo.utils when we dropped
timeutils module from our tree.

Change-Id: I4093a3c8faf3da5790ee31fdebec0b8ed70f84f3

stackforge/networking-bigswitch,wolverineav/networking-bigswitch,wolverineav/networking-bigswitch,stackforge/networking-bigswitch,openstack/networking-bigswitch,openstack/networking-bigswitch