Instead of raising ValueError, which is used all the time, except
DisconnectedSoftwareTree exception.

It allows to have selective detection of software instance problem and allows
to test what exactly was told.

Treat tree of software instance as graph and simulate requested changes.
Before doing any real changes check if graph is still connected.

Add utility method to software instance class in order to use collections
module and have fast graph walking.

Sometimes file grown, but because of previous cache flush. So just try to read
it and check few times if expected_text is found inside.

The order of steps is important in order to trigger underlying bug.

Reusing existing code made test unreadable. For now the only safe way to know
what is going on is to just define what shall be done.

Note: Currently it fails, maybe some work will be required.

It proves up to the end -- that communication with supervisord was done and
that service was correctly started.

Additionaly fix small issue with test assertions.

Mock netifaces in order to simulate real behaviour of machine. Add another
part of "persistence", which allows simulation of network stack.

Make test more flexible and controlallbe by exposing groups.

Additionally add required "persistence" to test.