Bump minimum oslo.config version to 1.2.1

oslo.config-1.2.1 has a required fix for interaction
with argparse, which effects trove and glance at least.
Related patch: I8d1b52e5390295726eb49af32eb7cab14c29592c

Change-Id: Icf7fa53834ddbd5a480984da9dbc39a207005b21
Merge "Bump minimum oslo.config version to 1.2.1"