Fix pass incorrect volume_size while building bdm

If specify device_name, snapshot_id, delete_on_termination=True
and not specify volume_size for bdm while create a server,
heat will pass the bdm such as {'vdd': '1234:snap:0:True'} to
novaclient, the volume_size is incorrect,
nova won't create a volume from the snapshot because the
volume_size=0. If not specify volume_size, just to pass ''.

Change-Id: Ic1407671cc7bc6695bcf8e0c9f86fbdbaf245658
Closes-Bug: #1335728
Merge "Fix pass incorrect volume_size while building bdm"