Add default value to agent create_or_update_agent new param
Add default value to agent create_or_update_agent new param

Commit Ifc88dfb3041aa07b197f395172b69399796ba46a recently added a
new agent_timestamp parameter to create_or_update_agent.
As some other projects use this api, the change should have been
backwards compatible.
Adding default to None will solve this issue.

Closes-Bug: #1833589
Change-Id: I2d17f8eaf4d74ae2081f82e3c1a09bdc80b1d701

openstack/neutron,mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,mahak/neutron