Add DB2 support to nova on redhat platform

Add three new attributes
default["openstack"]["compute"]["platform"]["mysql_python_packages"]
default["openstack"]["compute"]["platform"]["db2_python_packages"]
default["openstack"]["compute"]["platform"]["postgresql_python_packages"]
and add install DB python packages block in nova-common recipe

Change-Id: Ia862c821102119a784c4d8869418253e4ccdb137
Implements: blueprint db2-support
Merge "Add DB2 support to nova on redhat platform"