Update AlastriaPublicKeyRegistry.sol to work with publicKey hashes

[rober12]

Update of the publicKeyRegistry to make the functions work with the hashes (keccak256) of the public keys instead of the public keys in the clear (bytes32)

For example,

• https://github.com/alastria/alastriaID-truffle-contracts/blob/master/contracts/registry/AlastriaPublicKeyRegistry.sol#L26
• https://github.com/alastria/alastriaID-truffle-contracts/blob/master/contracts/registry/AlastriaPublicKeyRegistry.sol#L86

However, reviewing the whole implementation of the smart contract.