Add DB2 support to glance on redhat platform

Add a new attribute
default["openstack"]["image"]["platform"]["db2_python_packages"]
and fix an attribute typo in registry recipe

Change-Id: Ic84eefd460d1117c60570394e3b402a09a0e3016
Implements: blueprint db2-support
Merge "Add DB2 support to glance on redhat platform"