Add oslo.serialization for latest keystoneclient

Keystone tests run against keystoneclient master which added
a new dependecy on oslo.serialization

Conflicts:
	requirements.txt

Change-Id: I00f66b0f3c8e9f688429ee78bd51608a4dcac140
(cherry picked from commit a96bd7fb9c6cc75a3cff492c5f7a3097fe88cd99)