When the minion's key is overwritten with bad data, log a useful message
instead of traceback. Handle the error in a consistant way accross
salt.minion, salt.channel.client, and salt.crypt.

Allows event to be handled after the salt-minion service has received
a SIGTERM. Previously once the signal handler was entered, the ioloop
would no longer run. If there are events on the minion event bus that
needs processing, they would not be handled.

Moves the MinionManager stop() functionality to an async function and
allows the ioloop to run and clear any waiting events and returns to
masters.

Adds test for calling MinionManager stop() to test new functionality to
allow events to be processed as minion is stopping.

Sets up a MinionManager instance with a running event bus, then calls
the stop function and immeadiately sends a test message on the event bus
and reads it back to check that works once the stop() function has been
called.

Then checks that the usual functions to destroy the minion etc have
also been called.

Ignores warning from pylint about self.io_loop.add_callback() not being
callable - it clearly is as stop_async gets called.

Sets up the short sock_dir path in the test. Previously setting it in
conftest.py was breaking another test because I'd done it as a Path,
not a string, but I want to avoid changin behaviour of other tests, so
setting it locally to the test.