Sudo functions for Sparta testnet

[mnaamani]

During testnet we need sudo key to performs following actions:

• Gracefully end election
• Disable/Enable council election from starting automatically
• Start an election
• Set a new council
• Set election parameters
• Set new runtime

[mnaamani]

done in #22

[siman]

@mnaamani what do you think about adding such method to sudo set_stage(stage_name: Vec<u8>, ends_at: BlockNumber)?