salt-minion service should fail to start if master rejects key

salt-minion bails if the Salt master rejects the minion's key, but by this
point the Upstart job has emitted the start/running signal, so Puppet thinks
the service is up, and it executes things that require it, like various
Trebuchet operations. These usually hang.

So, perform a salt-call test.ping before starting salt-minion.

Change-Id: I78da7fec86b7991f7c1119fa59dde0471a4471cb