Add oslo.serialization to heat requirements
Add oslo.serialization to heat requirements

This patch add oslo.serialization to heat requirements.txt.
It's required for patch: Ibd64b3607721487be5b0562cdb54e6c76db51149
where this library is used.

Change-Id: If417206bd5f5df892d44db3b899bc87f18510d79
Closes-Bug: #1477908

redhat-openstack/heat,redhat-openstack/heat