Add a Python directory for RPC code to keep it at one place

Move rpc.py to rpc/node.py and modify imports in existing code.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>
Move Transport from luxi.py to a separate module

Also create a new module for RPC errors.
This allows it to be reused for other clients as well.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>
Split Luxi Client into a generic and a specific part

The generic part will be reused in WConfd.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>
Rename CallLuxiMethod to CallRPCMethod

Also update error messages and testing code to refer to RPC instead of
LUXI.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>
Separate the LUXI protocol version from the generic client

This allows other daemons and their clients (such as WconfD) to use a
different versioning sequence of their protocols.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>