Users can provide deployed SWAP contract

[vojtechsimetka]

Problem

Right now each time Swarm is started with swap flag, new SWAP cheque contract is deployed.

Task

• Add a flag that allows users to provide a SWAP contract address
• Programatically validate the provided contract
• If incorrect, raise fatal error
• If no such flag is provided, the contract address should be retrieved from the nodes storage. If does not exist the contract should still be automatically deployed.

[mortelli]

See also #1599.

[mortelli]

for clarification purposes: could someone please document here (or there) the difference between this issue and #1599?

relevant PR: #1718