Remove explicit set of CONF.os_region_name in mapr plugin tests

The explicit set of os_region_name to None breaks subsequent
unit tests running in the same worker process that attempt to
override os_region_name.  Any setting of os_region_name in unit
tests should use the set_override or set_default methods available
in oslo.config.

Close-bug: 1384875
Change-Id: Iea2eb58dc3d8e8667795495c6cc1f68538d524b6