Document volume properties

This adds documentation to the volume and volument attachment
properties.

Closes-Bug: #1224158
Change-Id: I22b4c8cb301b5de63418fd029ddf7f31087dfb8b
Add error handling to property value resolution

Fixes bug 1224746

Change-Id: I990de0159e0fb49e52afdaab546f8d74372422bf
Merge "Add error handling to property value resolution"
Merge "Document volume properties"