software_instance was not available anymore.

During finding root software instance not full tree is analysed. In order to
have more complete graph just add some nodes which shall be. They will be
mostly leaf nodes, so they do not impact graph connectivity nor cycleism.

Root software instance requests shall be disallowed, period.

Graphs were not cyclic.

Moreover update existing tests in order to simulate real cases.

Even more -- return True in case if all was ok.

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.

success

The promises now create a file and the test assert if it
exists

This promise check two suceeding promises on the same partition

This fix make the TestSlapgridCPWithMaster.test_stderr_has_been_sent
pass