This fix make the TestSlapgridCPWithMaster.test_stderr_has_been_sent
pass

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.

Conflicts:
	slapos/tests/slapgrid.py

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.

This solve the problem of software release which doesn't provide
any promise

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.

and white spaces)

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.