functional: close DBUS connections after running systemd commands
functional: close DBUS connections after running systemd commands

As described in https://github.com/coreos/fleet/pull/1704, functional
test has a bug hitting an upper limit of ~40 tests in total. That's
actually an issue of DBUS connections remaining opened even after
nspawn containers got successfully terminated. That's why the number
of unix sockets grows up to 256.

  $ netstat -nap | grep /var/run/dbus/system_bus_socket | wc -l
  256

From that moment on, functional test hangs mysteriously. Sometimes
users could see errors like "The maximum number of active connections
for UID 0 has been reached."

Its reason is that every DBUS connection was never closed. The more
tests we add, the more stale DBUS connections we have. This bug has
existed since the beginning.

Fix it by adding conn.Close() after running systemd commands.

jonboulle/fleet,giantswarm/fleet,endocode/fleet,endocode/fleet,coreos/fleet,jonboulle/fleet,coreos/fleet,dongsupark/fleet,endocode/fleet,giantswarm/fleet,dongsupark/fleet,dongsupark/fleet,coreos/fleet,jonboulle/fleet,giantswarm/fleet