Skip none value attributes for ec2 image bdm output.

Fix crash in _format_block_device_mapping when there are None values
for formatting attributes in the mapping.

Change-Id: I0ebdb844d75cdf1580cc88e1ff40fc21aff96f21
Closes-Bug: #1323403
VMware: Remove ds_util.build_datastore_path()

Convert all remaining users of build_datastore_path to use the
DatastorePath class, and remove build_datastore_path() and its tests.

Change-Id: I73a166a48c0038743213e4feaa95834f9ebc8fbf
Merge "Skip none value attributes for ec2 image bdm output."
Merge "VMware: Remove ds_util.build_datastore_path()"