Move iallocator script execution to ganeti-noded

Currently the iallocator execution takes place in the master, which is a
violation of the current architecture, and will create problems with a
threaded master daemon.

This patch moves the execution to the backend, similar to the hooks
runner, by:
  - introducing a new class that handles the execution in the backend
    (and could be used also for listing the allocators, etc.)
  - introducing a new rpc call
  - replacing the actual execution in IAllocator.Run() with a rpc call

This passes burnin with the dumb allocator

Reviewed-by: imsnah