Native support for cloud

[alecandido]

Currently, the backends' infrastructure hard-codes the assumption that the client and server are running on the same machine.

Despite an executable on the same machine will always be required to spawn the backend (to absolutely avoid adding backends details into qibo-core), in principle that executable could launch the actual backend on a different machine, and then quit, just returning the address of the true server to the client.

This type of server could be directly supported by qibo-core itself (and partly they have to, because otherwise all clients will look in the same place, i.e. localhost).