Bump oslotest minimum requirements to 1.2.0

Since it was recently released, lets bump minimum version of
oslotest to 1.2.0. Some keystone related projects plan on
leveraging code that is 1.2.0 specific.

Ref: https://review.openstack.org/#/c/130045/

Change-Id: I151d02fe0fa4ef7355847c779b3cbecc87e0c242
Merge "Bump oslotest minimum requirements to 1.2.0"