Fixed problem with the NO_DATA type in sqlalchemy
Fixed problem with the NO_DATA type in sqlalchemy

sqlalchemy storage backend uses a special type _NO_DATA_ to be able to
store frames where no data where collected.
Since this change, the data exposed by get_time_frame was leaking
NO_DATA frames, they should be masked.

Change-Id: Id682d92f82a4db8419f9d98a7b50ee5a65e7816f

openstack/cloudkitty,muraliselva10/cloudkitty,FNST-OpenStack/cloudkitty,muraliselva10/cloudkitty,stackforge/cloudkitty,stackforge/cloudkitty,FNST-OpenStack/cloudkitty,openstack/cloudkitty