payload_signer: update to signature version two

Do a key rotation and have the update engine use keyslot 2.
delta_performer: disable metadata verification

metadata verification has no method for doing key-rotation of the
payload. Since it is designed primarily to protect against a man in the
middle attacker disable it for now until we can come up with a
key-rotation strategy.
Merge branch 'multikey' of git://github.com/philips/update_engine into version2

Conflicts:
	payload_signer.cc