Preserve the configured log level

The Log handler fixture manipulates the loggers log level instead
of just setting the log level on it's own Handler.

If the ``level`` parameter explicitly set to None,
it will not change the level, configured by tempest.

Note: It is a reported fixtures issue.
https://bugs.launchpad.net/python-fixtures/+bug/1244256

Change-Id: Ibb6e1e6bba290ac6941fde184eacd241fe078521
Raise specific exceptions on tearDownClass failure

This commit removes the exception handling from the base network api
test case tearDownClass(). Previously every delete operation that was
used in tearDownClass() was inside a try except block that would log
any exception, and the continue with the rest of the cleanup until the
end of the function where it would raise another exception without
any information. This approach just masked errors when they happened
and makes it harder to debug what is failing. Instead the exception
should just be raised directly.

Change-Id: I20b7219bdf2f1e06445e5bde3abe144089fc06e2
Merge "Raise specific exceptions on tearDownClass failure"
Merge "Preserve the configured log level"