In some cases, guest agents may leave temporary config files

The code in these three places manipulates the configuration files. I
was (admittedly) having more than the acceptable dose of failures in
execute_with_timeout() and one of them that failed was the move of the
my.cnf file. And this left a turd in /tmp. The failures I was having
with execute_with_timeout relate to other tests that were improperly
mocking/unmocking and some of those problems have now been fixed.

Found that the same kind of thing can happen in cassandra and
mongo. Since default umask is 664, this leaves a file with somewhat
questionable permissions and potentially (at least for MySQL) some
passwords in /tmp.

The files will now be deleted.

Some test cases have been added to exercise the code paths(s) in
mongo, cassandra and mysql. The tests verify that the files are
actually deleted by mocking the os.unlink call. setUp and tearDown
have been updated in each case to make sure that the monkey patching
is properly reversed.

Change-Id: I93302fc5b1b18c9dd7116370a945b94d1824c75a
Closes-Bug: #1354136
Merge "In some cases, guest agents may leave temporary config files"