Enable cryptographic signing of messages published from the master.

Create a new master configuration option called 'sign_pub_messages' which causes the master to cryptographically sign all messages published to its event bus, and minions then verify that signature before acting on the message.

Note that to facilitate interoperability with masters and minions that are different versions, if sign_pub_messages is True but a message is received by a minion with no signature, it will still be accepted, and a warning message will be logged. Conversely, if sign_pub_messages is False, but a minion receives a signed message it will be accepted, the signature will not be checked, and a warning message will be logged.  This behavior will go away in Salt 0.17.6 (or Hydrogen RC1, whichever comes first) and these two situations will cause minion to throw an exception and
drop the message.