fix copying the mysql logs on completion

after telling jay that we were creating and archiving mysql slow
logs on every run, I was found to be a liar. This turns out to be
because we did shell expansion at the wrong point and got the
following in cleanup:

+ sudo cp '/var/log/mysql/*' ....-devstack-vm-full/logs/mysql/
cp: cannot stat `/var/log/mysql/*': No such file or directory

Do a cp -a instead to not fail on shell expansion

Change-Id: Ifd20ba17f7ff7d528697fbadd9f1d0178e7e7767