DescribeInstances in ec2 shows wrong image-message

If one instance boots from a volume,
the value of 'image_id' by 'DescribeInstances' in ec2,
will be a wrong number.

The value of 'image_ref' for this type of instances, is '', not None.
So the original protected codes can't take efforts.

Change-Id: I8c5e1c626f2b7873e7720f3615db1ee1ec6c681a
Closes-Bug: #1278696
Fix bad param name in method docstring

Fix tiny nits in a couple of methods.

Change-Id: I327bd78d232ffd5c8d2d1dd60724397430192a61
Remove legacy block device usage in ec2 API

This patch makes the single place that would get the block device
mappings from the database, not convert objects to legacy format,

Also fixes fake values in tests.

Change-Id: I77ecd8782deeec42963b5b1dd16bd85923b4d261
Merge "Remove legacy block device usage in ec2 API"
Merge "Fix bad param name in method docstring"
Merge "DescribeInstances in ec2 shows wrong image-message"