Refactored integration test of openstacksdk logging feature

Instead of asserting that openstacksdk created a logging file,
we read the contents of that log file now. The benefit is, that
this integration test gives users an idea how to actually use the
logging feature and also read the contents of the log file.

Change-Id: I9e3c75dda4c1c920937c9780fbdcbe3caeb4d6ad