heat engine : DBInstance don't pass credentials to cfn-init

Don't create user/accesskey and pass them to cfn-init as resource
metadata for internal nested stacks is not available via the cfn API
so passing them causes an error in the engine logs (and creates some
unnecessary keystone credentials)

fixes bug 1097362

Change-Id: If2fe484d834e3e5730b21a18c363a5bf5a4c45f6
Signed-off-by: Steven Hardy <shardy@redhat.com>