cli: "result list": Add support for listing rolemaps

The new schema introduces rolemaps, which map each role in a benchmark
to a node where it executes. Let's add support for listing these objects
using "result list --object rolemap".

rolemaps do not have a friendly name, so their user-facing ID is always
"rm<id>", where <id> is the numerical id in the database.

The rolemap consists of a set of rmdescs which map a benchmark/role to a
node.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>