Convert rpc results to a custom type

For a long time we had the problem that both RPC-layer errors and
results from the remote node share the same "valuespace". This is
because we shouldn't raise an exception when only one node failed
(and lose the results from the other nodes).

This patch attempts to address this problem by returning a special
object from RPC calls, which separates the rpc-layer status and the
remote results into different attributes.

All the users of rpc (mainly cmdlib, but also bootstrap and the
HooksMaster in mcpu) have been converted to this new model. The code has
changed from, e.g. for boolean return types:

  if not self.rpc.call_...

to

  result = self.rpc.call_
  if result.failed or not result.data:
     ^ rpc-layer error    |
                          - result payload

While this is slightly more complicated, it will allow cleaner checks in
the future; right now the code is just a plain port, without
optimizations.

There's also a "result.Raise()" which raises an OpExecError if the
rpc-layer had errors.

One side-effect of the patch is that now all return types from the
rpc.call_ functions are of either RpcResult (single-node) or dicts of
(node name, RpcResult); previously, some functions were returning
different object types based on error status.

The code passes burnin (after many retries :).

Reviewed-by: imsnah