Fix pyghmi behavior on BMC NACK
Fix pyghmi behavior on BMC NACK

When BMC sent NACK, pyghmi incorrectly attempted to construct new payload
by concatenating a tuple and str.  Fix by converting the tuple to
str prior to sending.

Change-Id: Ia07384651870bf01e7dc24757e1e00018ab84556

stackforge/pyghmi,benoit-canet/pyghmi,openstack/pyghmi,openstack/pyghmi