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.

Modifies systemd_service.{restart,stop} to default to using
no_block=True when the service being stopped or restarted is the
salt-minion.

If you don't pass no_block, the minion blocks waiting for systemd to
restart the service, while systemd is waiting for the minion to exit.
Eventually, after systemd hits its timeout it will kill the salt minion
processes.

Behaviour for other services should remain the same and the functions
will still honour the value of no_block if passed as an argument.

Skips the test_operation_no_block_default test on Amazon Linux 2 as the
CI conatiner for that distro doesn't have a complete systemd setup.

This reverts commit 4c0e552.