unit: invalid configurations accepted when using `processes/spare/0`

When using spare processes field set to 0, the router doesn’t actually restart the app but just validate the config. This makes router accept invalid configurations and hang the application in unexpected ways.

Eg.:

$ curl --unix-socket $UNITSOCK 127.0/config/applications/hello
{
	"type": "python",
	"path": "/vagrant/src/unit-configs/ok/python/hello",
	"module": "wsgi",
	"processes": {
		"spare": 0
	}
}

$ curl --unix-socket $UNITSOCK -XPUT 127.0/config/applications/hello/working_directory --data-binary '"/non-existant-dir"'
{
	"success": "Reconfiguration done."
}

$ curl localhost:6666
.
hangs here...
^C
$

I got this problem in tests (that set spare to zero by default), tests were passing when actually they should not… @andrey-zelenkov