Fallback gas price for native-to-erc20 mode

[akolotov]

At this moment it is not possible to set the fallback gas price for native-to-erc20:

https://github.com/poanetwork/poa-bridge-contracts/blob/048f51e0e8e7ce4213b727c0982aadab6c994db9/deploy/src/loadEnv.js#L225

https://github.com/poanetwork/poa-bridge-contracts/blob/048f51e0e8e7ce4213b727c0982aadab6c994db9/contracts/upgradeable_contracts/native_to_erc20/HomeBridgeNativeToErc.sol#L104

For scenarios when the validators addresses are whitelisted to be allowed send transactions with zero gas price we need to allow configure HOME_GAS_PRICE as 0.