Clean /etc/mysql when calling clean.sh

The clean.sh script should also remove the /etc/mysql
directory. It contains information from the old
devstack installation and may conflict with the further
one. apt-get purge does not remove it since the
directory is not empty.

Change-Id: I885345a2311851d8746abe42e44300ecd4f6e08a
Merge "Clean /etc/mysql when calling clean.sh"