Proposal: rename volumes

[buchanae]

From #95

Proposed alternatives:

• shared_directories
• common_directories
• common_data
• disk_paths
• directories

[pditommaso]

Other proposals:

• mount_points
• mount_paths

[buchanae]

None of the proposed names are strike me as being more clear than volumes, and the term volumes is fairly close to the way Docker uses it. Also, the volume field specification has been simplified and the documentation improve substantially.

Feel free to suggest more names or a better description, but I'd like to take this off the TODO list.