Provide an ability to deploy AMB with existing implementation contracts

[akolotov]

It is obvious that for further AMB adoption it could be a situation when one chain concentrates several AMB contracts of the same nature. Even for now there are two bridges ETH-xDai and ETH-POA as so the Foreign chain for them is the Ethereum Mainnet.

It means that all these bridges could have unique addresses but will share the same implementation. Since the bridge uses the proxy contract template, in order to safe gas and storage, it makes sense to have one implementation contract for all the bridges.

Deployment script does not support such option yet, so consider to implement a new configuration parameters like *_BRIDGE_IMPLEMENTATION and *_VALIDATORS_IMPLEMENTATIONS where the corresponding deployed contracts could be specified. If one of the parameters is configured, the scripts will skip deployment of the contracts and will use existing one instead.

[akolotov]

Initial implementation of suggested changes has been started in https://github.com/poanetwork/tokenbridge-contracts/tree/existing-implementations