RAPI client: Rename Get{Node,Instance}Info, add new GetInstanceInfo

GetInstanceInfo should return the resource /2/instances/$name/info,
but so far it returns /2/instances/$name. The same applies to
GetNodeInfo, which returns /2/nodes/$name. Both names are stripped
of their “Info” suffix and a new method, GetInstanceInfo, is added
for /2/instances/$name/info. There's no equivalent for nodes.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>